I'm just in the process of figuring out a similar system, but for under a trailer as a drawer. 4x8 and maybe 8-10" deep should keep plenty of ground clearance and make a place for the splitter, wing, and side skirts to go during transport. I'm leaning towards skate board bearings and square tube. Never thought about conveyor track. Now it has me thinking i should use the skate board wheels for the larger diameter and hopefully less problems with grit/dirt or whatever else might get kicked up in there.
I've used a variant of this design on every truck I've owned for the last 25 years. I use a sheet of 3/4 ply and four 2x4s on edge for the platform. I mount three pairs of 4" tall non swiveling casters using carriage bolts through the top. They wind up leaving about 1/4" of clearance to the bed. Almost every hobby I've been involved in needed a folding table. The table invariably would end up on the bottom of the pile so I'd wind up unloading everything, getting to the table, then loading everything on table. With this setup the table is always easy to get to.
I should probably add that when I built this a few months ago, plywood was a lot cheaper. I think you could probably still build it for under about $3500, though.
In reply to JG Pasterjak :
These days you would simply trade the Tundra for the wood, but then what would you use the wood for?