You are partly back on Imbert 1926!
If you can afford to "invest" on easier ash removal,
put a "grill" of 12mm angleirons 30mm abowe the siporex plate!
The angleirons (pref. SS) should have short bits of square tube tightly attatched (welded) on their ends.
Angle irons tightly inside the square-bits!
The square-bits in both ends = broder slots,
square-bits in each second end = narrower slots.
This keeps them uppright in a horizontal ring of U-bar.
The U-bar is bent to a wheel where the profile is open horizontally inwards toward the ring-center.
You can weld the U-profile together, and cut one only slot in the profile, to be facing downwards. One side of the profile (downward) gets a slot broad to fit and pass the square bit in the end of the anglebars.
You put in the prepared angleirons, one by one, the shortest first, on each side of the cut-out slot.
The last one comes in the center.
Then you shake the assemly (turned upside down) (leaning on a small diameter bucket) and turn whole the angle-iron package 90 degrees in respect to the U-profile ring, which carries them!
The angle irons are easy to change, if you leave enough play inside the U-profile ring...