pardon my poor English and Photoshop skill.
Basic idea is to make a ceramic barrel.
The barrel will sit on the original fire grate
Coal will be placed inside the ceramic barrel.
Above coal there is a ceramic(or casting iron) fire reflector
0.5" to 1" above the fire reflector, sitting a pizza stone
A little bigger ceramic plate sits on top of the barrel as cover.
In the center of the cover, there is an air outlet.
The bottom of barrel can be ceramic too, but with small holes
This way original fire grate can be removed.
For high temperature grilling, just replace top cover with original grill
If I have size of original fire grate, and height the barreal can reach, I will make a few samples