I am doing something similar with laminated oak boards.
The idea of insulation on the bottom is good but one must take into consideration the grit that is going
to splash up on that soft underneath and how well it stays adhered. I used two coats of DP40 epoxy primer on the bottom with one coat of Pro Form Gravel Guard European style. On the top I used to layers of West System Epoxy and well sanded, then four coats of Sherwin Williams Marine Spar Varnish. The stock strips on my bed were just under 80" and I powder coated them silver vein. I used stainless fasteners.
cheers