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If that truck bed is 4' X 8', you could use a 1" thick 4' X 8' sheet of extruded polystyrene, aka Styrofoam, from Home Depot, etc.. You could fiberglass both sides of it, for a very stiff but light panel. Or, with proper adhesive, you could skin it with upper and lower sheets of 2mm thick Coroplast or other suitable sheet plastic. This becomes a sandwich panel of incredible strength and stiffness. Styrofoam is nice to work with, as it sands and sculpts easily.
You want stiffness, lest the panel flop around in turbulence at highway speed. With stressed-skin panels like this, when you double the thickness between load-bearing skins, you get an eight-fold increase in stiffness, which increases with the cube of the thickness between flanges.
To keep prying eyes out, you probably don't want the panel to be transparent or even opaque. If that's not a problem, visit a greenhouse supply store, and pick of a sheet of duo-pane polycarbonate, which is extruded and just like Coroplast, but transparent and virtually impact-proof.
I doubt that lip does much if anything aerodynamically, but looks nice and probably sells a lot of those panels.
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