I'm late to the party... I thought I had written about this on my build thread, but can't find any mention of it there now. Anyway, this photo basically sums it up:
Do the template in two pieces. Rig the uncut template material as high up the wall as you can, so that its edge is flush to the wall, and then use a fixed-length stick and a pencil to translate the curve of the roof onto the template below. Then cut along the line and hold the newly cut template against the roof to check the fit. The photo shows my straight edge and pencil clamped in approximately the way I held them.
Depending on how tightly you want the template to fit, you might choose to repeat the procedure. After you have the top part of the template made you can attach a second piece of material below to make a full-height template.