If you're building a wood box for a trailer, I suggest 5/8" or 3/4" green treated plywood.

K,

I built a wood box for a 5'x8' utility trailer. I used 2 4'x8' sheets of plywood. Very easy, simple construction.

If I were to decide to make a rack for canoes, It would just need pockets for 2x4s would would be bolted to crosspiece 2x4s. This would allow me to carry 2 canoes side by side. Plus I can carry 2-3 canoes on my van. Since my days of carrying canoes for a small army have come to an end, I don't need to build that capacity on my trailer.