Sorry... I really appreciate all the build details that some people provide, but I guess I'm too lazy for a build thread... so far. Plus... I don't think I'd recommend doing what I did. The curves and facets are nice, but a PITA. Also, the amount of fiberglass dust generated was extreme. I thought that since a rough finish was ok with me, the sanding would be minimal, but that wasn't the case.
To answer your questions: It is wet layup epoxy/fiberglass and carbon over PVC foam core. Built in pieces and then joints filled with fumed silica/epoxy, and more layup on the seams. The bottom is carbon and structural (no flatbed) and joined to the frame using poly cab mounts to create a pseudo 3-point pivot. If you have a truck with a fully boxed frame you don't need that pivoting stuff. It's big, over 9' plus a big cabover bed and 76" interior height.
What would I recommend? Simple shapes. And if doing wet layup, buy cloth that is as wide as the piece you are making if possible. It's amazing how obvious a .5 mm discontinuity is on a flat surface.
Based on what I've seen of other builds, FRP skins glued to foam looks like the easiest way to go. There are manufacturers in Oz that get away with very simple edge joining also... basically a rebate/butt joint with glue.
I just got back from it's first real camping trip to the Big Bend area, and it performed very well on road and off. I still need to build under bed storage... waiting for it to get warm enough for epoxy work.