What is thickest plywood you can use? Its 1/2 now, with age has become way to flimsy. Id love to use your parts to get her back on road. My kids have kids, and probably would end up with it for there family! I hope your parts are picked up, If not Id be up for some experimenting with first runs.
No matter how thick it is, a large expanse of unsupported plywood is likely to sag over time. I've made the TrailTop parts to accept 1/4" panels; for an application like a flat cover for a trailer like yours, or the one I showed for the Jeep-tub trailer, the way you'd assemble the top is with 1x2 cross-ribs of either pine or poplar to strengthen everything and prevent the plywood from sagging. Here's how everything would typically go together:
The cross-rib is bonded to the plywood and the TrailTop side rail; epoxy would be recommended for the wood-to-fiberglass bond, and could also be used for the wood-to-wood bond, although since wood glue is less expensive than epoxy, wood glue could be used for the wood-to-wood bond as shown in yellow above. Wood screws (or bolts if desired) would also be used to secure the plywood to the TrailTop parts and the plywood to the ribs. Weather sealant (could also be epoxy, or something like automotive seam sealer) would be used to seal the plywood to the TrailTop side rails.
A mix-as-it's-applied epoxy like West System Six10, which is designed to fit in a standard caulk gun and has a tip that mixes the epoxy as it's squeezed out, could be used for all joints and as the sealant and offers great strength in all bonds and easy application. (
http://www.westsystem.com/ss/six10-thickened-epoxy-adhesive/)
That's probably more than you wanted in an answer to your "how thick" question, but I figured I might as well cover all of the details
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