How about simpler? I've checked 1/4 inch plywood this afternoon and I agree with my engineer friend. It has plenty of rigidity/strength and the cost factor is favorable. I think something like the quick and crude drawing below might be a starting point. The piece would sit in the trough below the radio and be held on at the bottom by screws/nuts...similar to the way the radio was intended to be attached. Two 3.3" diameter speakers would sit side by side and a foam pad (not shown in the drawing would sit at the top to prevent rattling.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
