I’ll be straight forward here and just dump whatever comes to my mind.
Your drawing shows about the flow path I’m running every day. Minus the grate.
With a permanent bottom sediment it seems about a 1" void stays open where gas and char can travel. I would like to see a bit more space in the bottom corners though. The flow path will create its own rounded corners and things look a little narrow in your drawing.
Also, since nothing will be reachable from the top, I’m with Garry here. I wouldn’t dare go without at least a 1 1/4" cap on the side, close to the bottom. In case of plugging you could empty with a drillbit/auger.
I vote for an angle iron grate, resting on tabs, movable sideways and with a bottom fixed rod, like Wayne uses, for shaking.
I’m a little hesitant to a horisontal cyclone. It would probably work just fine with fixed rpm and load, like running a generator, but what happens with a lot of variation in flow? I don’t know.
Even if I’ve used only vertical cyclones they haven’t been able to handle big char (>1/2") very good. For some reason they collect ash, soot and small char just fine but quite a lot of the big char settles later on in my condensation tank.
Oh, and in this limited space I think a double walled, heavily finned cyclone would shed even more heat than tubing. Also it’s maintence free, no cleaning needed for heat transfer.
Edit: Obviously I type very slowly. Several posts popped up after I started this one