we were workin in MIAMI...fla
doing a lawyers office....
while on the job.....
we took a piece of ply wood and transfered the roof's curve (shape)
by getting it close then scribing it exactly to fit... giving us a frame
that sat
on .... but curved around the
edge so the weight gets distributed
to the cars
full frame. just like the curved bridges do made from stone.
i tell you this....because we carried
unbelievable weight 
on this
frame
20 sheets of sheet rock or plywood like nuthin...
and it was completely carpeted so it didnt even leave a scratch....no dents period
and it was fairly light weight.
anyone can make a car rack like this.... with a bunch of bungie cords
to anchor it down to the body you can carry anything that way.