Something I've seen that's a good low-tech, one-moving-part solution is to run a piece of steel pipe across the back using a piece of slightly larger PVC as a sleeve over the pipe. This looks cheap and doesn't have any fancy bearings or other parts to rust or jam with dirt, corrode due to road salt, or bet messed up with other crap we pick up. I've seen this concept used various sized steel bars up to 3" or 4" in diameter. Most recently I saw a trailer on Monday using what looked like 3/4" galvanized water pipe and 1" PVC.
That said, I've heard folks complain about single-bar full-width rollers warping, however this design would probably still function even if the steel bar were warped as its the flexible PVC that's moving.
Go walk around someplace like Hecla Junction and check out the trailers, you'll get to see lots of different types of rigs.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse