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.