Here's another material that's a lot cheaper (I think) and comes in a lot of colors. It's called Durabak, and it's used for heavy industrial applications like warehouse floors and boat docks, so you know it can take a beating.
http://www.cotelind.com/
You can request their literature, and they'll also send you samples of the stuff. It also goes right on the ply with a roller.
It's a matter of taste whether you like the texture of it--it seems to go on heavier than the bedliner stuff. But the stuff will actually fill and seal your seams where your roof meets the sides.