I would start with a 10”x10” HSS 11” long,
lay it on its side, cap one end with a light 1/8” plate.
Make a plate removable on the other end.
Cut a round hole on top for your burner,
Door hole front and flap door hole out back.
Inside dimensions no less that 9”wide x 10”deep x 5” high,
working opening 2" x 9"
adjust for draft shoes.
Put a shelf out front and a flap door out the back, 2”x2” will do.
Recommendation: use a factory burner/venture only,
I have manufactured my own and they worked just fine for me
but if you don’t tweak or know how they wont work for you.
I hope that helps