Simplest system is a short bit of galvanised pipe (enough to span the width from the outside trailer frame to the extent of the boat), an ell, and a longer length of pipe as an upright, with a piece of PVC pipe over the upright to create a roller. Use a couple of U-bolts to clamp it to a trailer frame member, then repeat on the other side.
I've got these things on my trailer:
and they seem to work pretty well, as long as you don't hit them too hard. They require a bit of straightening if you do (the box section angles tend to be a bit wimpy.)
They're really handy when recovering solo, as you only need to worry about getting the front of the boat in place - the back takes care of itself, even with a bit of cross-swell.
Forgot to mention: they also make a handy post for hanging drysuits to keep them off the ground.