Avion truck campers - Hundreds of photos

D, here’s a quick idea to address your problem. I’ll expand on it if it interests you. 1. Clean all the dead wood out. 2. Make a full sized paper pattern carefully following the curves at the front. 3. Get either an 1.5” piece of coosa board or some thinner sheets you can bond together. 4. Cut out the pattern on the coosa. 5. Shape the outside edges of the coosa to roughly match the curved molding at the outside of the cabover. 6. Make a thickened epoxy putty compound and butter the outside edge of the coosa then press the coosa firmly into the bed space. The epoxy putty will fill the voids and bond everything nicely all around the edges, creating a rigid floor completely moisture proof and 30% lighter than plywood. You might do the same with sheets of plywood carefully epoxy coated before you install them. Much cheaper than coosa...
If this an approach that appeals to you, I’d be happy to expand on it. The devil’s in the details of course...