I'm building my own partially-enclosed trailer for windsurfing equipment and am looking for a good material to make the sides
of the box. Has to be zero-maintenance, tolerate a lot of water/moisture (including salt water), and be reasonably priced.
I figure 1/4-inch would be a good thickness although thinner might work.

Seems like corrugated panels are available everywhere but can't find flat ones!

You could string up hardware mesh, and apply your own fiberglass over
it..