Anyone tried using a small boat trailer to haul plywood and 2x4s?

Sure. Use ratcheting cargo straps and/or C-clamps.

Are you going to use this when you are not using the boat, or are you
converting a boat trailer frame to be used to haul materials?

If you are trying to get double duty from a boat trailer by occasionally
using it to tow, no problems. Just try not to overload the tongue or rear
end, depending on how the wheels were placed. They were placed to match the
center of gravity and balance point of the boat, and could be a bit off for
hauling.

If you are going to just use this trailer for hauling materials, you can cut
off stuff, and make flat decking to hold materials.

Either way, the trailer will serve the purpose provided you load it right,
and consider weight, balance points, and centers of gravity when carrying
things.

Remember weight limits, too, as a load of plywood can get close to weight
limits real quick.

Steve