I would use real wood or at the very least, steel. I used to work in the lumber industry, and everyone that has commented so far on it is exactly correct, it does a heck of a job withstanding the elements, however, it is very flimsy and brittle. In your defense, such a short piece of it (the length of a truck bed) it would be a little more stiff than say a 16' piece. Also, the plastic lumber seems to be awfully heavy. At least any of the stuff I have ever had any experience with.
If plastic is what you decide however, best of luck to you! Build it your way.
