I built, for a different purpose, a frame measuring 4' x 8' of 3 1/2 clear douglas fir 4" x 4" x 8' posts, and installed six industrial casters underneath it. I stacked on approximately 2200 pounds of oak lumber, and if I hitch it to the truck I can move it around. The entire assembly is done with lag bolts and washers, all 5/8" hardware, and casters rated 250 lbs. each (I had more wood than I anticipated, so the casters are overloaded). This has been in service for two years without any sign of weakness. Load stress noises occured for the first half hour of use.

The side frames are each composed of a full length 4 x 4 x 8. The three crossmembers are 4 x 4 x 4. Lag bolts draw the pieces together, and then the casters are installed at each corner and midpoints. I could easily have used two center cross members and two additional casters and increased the load bearing capacity to a full ton.

A larger version could readily accomodate a car body. The side rails are continuous support beams, and should be sized to support the rockers. It can be readily disassembled and stacked until the next use.