Heading north out of Denver last Sunday while a steady stream of weekend warriors headed south, we saw an interesting implementation. A fellow had his cabover Truck Camper mounted backwards on a single axle, ATV hauling-style, 5' x 9' [?] Utility Trailer with the ramps up, of course. There were no yellow Semi Truck straps evident, so I assumed this was an ingenious weekend rig he had devised with Camper held firmly in place some other way. The Trailer sidewalls were the usual ~1'-high angle iron.
Perhaps fixed or removable Camper Jacks would be one way to lift off a Camper or U-Haul box and still have use of a heavy duty Utility Trailer too. A fellow on this Forum did a sharp job of building a permanent box onto a Trailer with >1' sidewalls.
On the commercial, dual-axle Utility Trailers I've looked at so far to perhaps 'roll my own' CT box, the ~1' sidewalls were reportedly an integral part of the Trailer strength, not to be altered or removed. Heavier-duty Car Haulers have no sidewall angle iron, and permit more box design freedom. But, typically at a higher cost and weight.
On a Boat storage lot up the Interstate, we also saw 3 used U-Haul Trucks for sale, with rentals also. One had the box intact; the other 2 had no boxes. One had to wonder if the boxes were damaged or sold off separately? Perhaps the local U-Haul franchise or used Commercial Truck lot is a good place to start a box search...