Starboard

[chumbucket]

The problem is, Starboard, although UV stabile is not a good structural material to handle loads. Most plastics aren't unles they are rotationally molded or molded with some sort of strengthening gridwork to keep it from deflecting.
The Starboard material would work for this project, but it would need something attached to the underside to keep it from deflecting when stepped on it.
I used some Starboard for a smaller project and I can tell you that it's very nice and easy to work with. No special tools are required. Just basic woodworking tools. You can cut it with table saws, route the edges, pretty much anything similar to woodworking.
If you can find a plastics distributor in your area, they will generally sell it cheaper than the marine stores do. I bought mine at a plastics place here in Boston.