I've got a metal bed frame here which was halved and adapted to allow the two halves to move like the loading gates on a standard stock trailer
Never got round to putting the trailer together, so they're still kicking about
I'll have a look at them tomorrow and if they're still good you're welcome to have them. You'd just need some little brackets to hold them in place.
Ditto the plywood panels (bolted on, so easy to remove) and a tarp over the top to waterproof - easy, cheap and won't stop you using the trailer for anything else.