There is nothing wrong with 20 ga however if you don’t have a substructure and only rely on corner flanges, you risk denting at the centers.

I agree, if your fabricator uses a tubular steel subframe then 20 (.038) ga is fine.  All of the parts inside can be as thin as he is capable of welding.  The burner box can be made of thin gauge galvanized since it is covered with a door.