I watched a guy unload a 16v GTI from the back of a U-haul at the Portland ESCORT race 20 years ago. I am NOT making this up but he unloaded it using ramps made out of 2x6s nailed togehter, about 12' long, that were supported in the middle with JACK STANDS. Successfully unloaded it, I might add.
It also appeared that he had not tied it down in any way.
I think if the right ramp/tiedown system were found, it wouldn't be half bad. Heck - put an AC unit on the roof and sleep in it after the car is unloaded...
If a person wanted to build a box from scratch, it could actually be VERY cool. Put side doors on it where you need to crawl out of the car, or use a winch to drag it up onto steep interior ramps, to get the nose up and use some of the otherwise dead air space. Put storage in the area under the inclines, accessible through exterior hatches for jacks, tool boxes, etc. Mount tire racks on the upper inside portion of the rear doors, so they swing into more of that empty air.
Dick's right on about weight. If one started with an Isuzu or similar turbo-diesel "lowboy" chassis (with small diameter rear wheels), he's only use a small portion of it's load capacity.
K