You can take a sheet of 5/4 marine grade plywood to make a 1 sheet wood bed if you want, and route grooves in it or just mount the wear strips to the top. It has water proof glue, just make sure you finish all the edges.
Trex is nice stuff (I have a Gazebo I built with it for the floor) but it does sag, 16" is the absolute minimum support distance. However, it should be easy to buy a few more braces for your bed and put them in between the stock ones. Another thing to watch out for is that is seems to be much heavier than a 2x6 of similar length. And, it does scratch.
