Some guys I hunt with did something along the lines of what you are describing. They built a box on the front of the trailer with 2x4s and exterior plywood to fit all of their decoys in, and parked their wheelers in the back. They painted up the box with some marine-grade paint, and used it for three seasons before getting an enclosed rig. I think they had it bolted to the sides and through the floor with U-bolts.
It worked well, but it was a beast to haul. It was heavy and caught the wind badly. It was better behind a truck with a cap on the back, but even then, it was wider than the truck and still caught the wind. If I was going to do something along those lines, I would make sure it was no wider than my truck to cut down on drag.I would also build a v-nose section out on the tongue to cut the wind some, and for extra storage that won't eat up space in the bed.