If you ask your local printers, they may be able to give or sell you used Lytho aluminium sheets to make into your roofs.
A few notes based on my experiences:
1. Depending on the size of the printers, make sure your car is big enough to put them in. You shouldn't put them on the roof for safety reasons. Bring some duct tape to secure the sheets into a roll, but be very careful when undoing the tape as they whip open with considerable force.
2. Where possible, wear gloves. Edges can be VERY sharp
3. Score sheets on a straight edge with a Stanley knife and finish cut with substantial scissors or by bending back and forth at the score line.
4. I bent sheets by clamping into a B&D Workmate and gently pushing to a right angle.
5. You need to snip the corners to get the correct fit then pre-drill before nailing.
As ever, trial and error will get you a good finish after you have messed up the first time!!