I don't understand why you didn't build this around your proposed 3/8" plywood stiffener. One and done kind of operation. The Mylar worked perfectly. I priced it versus Peel-ply...much more economical. I'm missing 2 hatch-covers myself.
The plywood stiffener stayed back a couple inches all around so the hatch would fit level with the deck and gutters like the original hatches did The missing originals had the stiffener plywood back from the edges the same way The mylar does not absorb the amine blush like peal ply so it still has to be washed and sanded afterwards Thanks for watching
