We picked up an old 11' or 12' travel trailer that was being junked (freebie), and stripped it to the frame. Added some used steel trailer fenders, decked it with 3/4" plywood. Picked up a pair of vintage slot mags from a co-worker and a set of tires from a friends jeep. Had a couple of worn out 16x35r15 Boggers in the back yard, so we cut one up and skinned the fenders to make it look mean.
Since the picture we've removed the fold down ramp setup and went back to loading them forwards. It will need to be re-decked in the next year or two i'm sure, the plywood is starting to get a bit warped, but it's held up really well for the last few years.