Progress…
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I would still look at cutting it into pie pieces then have like 8 of them using the same mold. You could also look at a trough which is like a half a cylinder, the focal point is long and wide but for a cooker it might work better.
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yeah, I have made one of those for my vacuum tube…
Now that I have the program I can print out with my laser printer designs for different sizes, or use the string method to draw out a new pattern.
I have thought about using skids to make a trough design using my pattern for the shape…
Next steps for me is see what I can do with paper mache and bamboo, cheap and I have plenty of it.
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Has anyone tried this sort of thing? Aluminum foil on cardboard I think. Search: solar cooker
Rindert
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you can just use aluminum foil. You could probably make a frame out of wire.
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Yes, I have a friend that makes those for a refugee camps in Africa.
They can also be made also with car sunscreens. They have their downsides, they do get the job done, but to improve their efficency, use of a plastic turkey cooker bag helps to keep the heat in.
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I tried several different cardboard and foil designs. The one I liked best was like this:
Corrugated cardboard seemed like a pain to me. I was lucky and found a roll of heavy paper backed foil at a yard sale. It was not as strong as cardboard, but the conical shape held up pretty well. Not putting the flat disk at the bottom of the cone didn’t seem to matter. My experience was that the dull side of the foil worked as well or better than the shiney side for these cookers, though I wouldn’t have thought so ahead of time. I made a wire rack that fits inside the cone and holds your cooking container. It makes life easier. I painted a pint canning jar black, and put it inside a clear quart jar. Small lid inside a small ring, inside a large lid inside a large ring. with the two lids held together with a screw. Seal the screw to avoid a mess.
If you can find a metal bowl that fits inside a glass bowl and sits on the rim, with a glass lid on top, that works really well too. Dark color on the inner bowl will help.
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Thanks guys. I’m want do something like this to evaporate water out of apple cider this fall. This makes apple syrup. Just a way to process mass quantities of apples so bears don’t come into town. I have this four and a half gallon pot. Thinking I’ll cover it with a black or dark blue t-shirt.
Rindert
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Rindert,
These are all “cool” projects! As for the pot, I would just paint it with BBQ black or flat black enamel. You could certainly use any dark color cloth to see what works best!! Hopefully, it wouldn’t get hot enough to burn, but the insulating qualities of the cloth might work against you. 
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So, I am now trying out making paper crete. never done this before… I decided to run the cardboard thru a garbage disposal. I made a mess the first time around, but now I think I have the hang of this.
First wet the cardboard, then tear into small chunks, run thru the garbage disposal, then dump out on a bed sheet to capture the small cardboard fibers.
I now have the sand, cement, and now will process the cardboard and see what happens.
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I would probably put it in a box with a small air vent. If you want to stick with fabric look for a piece of landscape fabric from a landscaper. black plastic would work as well.
I wouldn’t want to paint my pot. 
I think the hardest part is going to be getting it reduced fast enough to keep the yeast naturally in the apples from working.
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I’m more worried about over heating. The syrup tastes like molasses if you over heat it.
There is a small vent hole in the lid. I’m going to try keeping that vent hole exposed while covering the rest with dark material. And I’ll see if can control it somehow by making the reflector more of less efficient. I have a notion that my muscle wire spring might help. I’ll do some trials beforehand using just water in the pot. I think if I can evaporate 3 gallons in 4 hours, in the middle of the day it I’ll then do the final stage on the cooking range.
Rindert
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I don’t think it is going to get quite that hot unless you have a reflector or larger collector With a quick google, the hottest temperature was 180F for a solar thermal water collector. You will lose some water.
For maple syrup, I have a pot similar to that but I think it is 12q and I heat it up to boiling temp with the lid on then move the lid off it part of the way, usually the pot handle is designed to hold the lid so the condensation doesn’t drip back in the pot. But it still takes longer then 4 hours.
The induction heater is about 2x as fast as the resistance heater for the same wattage for my use case.
Boiling is a surface area contact game, and pots aren’t really that great.
A faster way would be to heat air then pump the hot air through the pot. The bubbles greatly increase the surface area, and the water vapor gets trapped in the bubbles. You could run a stainless steel coil in it the pot. Some Maple syrup makers use steam to heat the syrup. Most fluted ‘sap’ pans that increase the surface area.
The more I think about it, your best bet, might be to create a solar hot air collectors like the ones out of old pallets, then push air so it heats the pot kind of like a warmer at a buffet. Not because it will boil faster, but you will need a collector of some sort, and they are more versatile, like you can use them to heat the garage during the winter. IF you had a pool then I would suggest like a solar hot water system.
If you use a collector like a trough or parabolic, or
If you want to get electronic, then you could use an arduino and control the temperature. then move the collector, but I would think about indirect heating, like heat bricks, or water or something that you can control better, but that increases the work and complicates the system, which I don’t think you are looking to do.
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It seems to me (in my admitted ignorance 
) that trying to get all that water through a little hole might slow the process. Could you use a sloped sheet, dark colored, facing the sun to collect the heat, and slowly run a film of sap down the surface? Maybe use a low-flow pump to cycle the sap? Lots of surface area per volume of sap, lots of ventilation to keep the humidity low and evaporation rate high, relatively. There might be issues with viscosity as the sap thickens, but then you could move to the range. The energy cost might be pretty low for the first part of the process. You might be able to speed it up a bit with additional reflectors, or a concentrator mirror.
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I 'm also thinking about a Fresnel lens because I could then use it to burn weeds also.
Rindert
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I had the screen from an old projection TV at one time but never figured out what to do with it and don’t remember what happened to it. Now I know what it could have been good for.
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Someone has a video where they are melting glass with one.
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