What about a nice double cab pickup? Takes the family comfortably and I know you can't lay them flat, but surely that would take 8'x4' sheets in some way?
Otherwise I would agree with the roof rack option. I have a v. old Grand Cherokee and all I did in the past was to tie two pieces of suitable sturdy 2.4m long soft wood on the cross bars and then load up and secure the sheet goods. Think I have had 3 x 18mm ply, 1 x 9mm mdf and 1 x 3mm hardboard on it at the same time and it was no problem, I would be happy to load 6 x 18mm ply on it (if I had the space to store it when I got home).
This is even though I did not do the cross bars any good when I used the Jeep to pick up 300m of damp feather edge, that was a step too far
Terry