Jolly Roger is correct, it's a topic that comes up from time to time. Not that this helps you any -- I've just failed to search my way to any of the existing threads I recall, and that's coming from a place of knowing it's there and having an inkling of what to search for!
Here's a photo showing my setup for tracing the curve of the roof. Basically I held a cardboard panel in a fixed position near the curve with a rough hack at the shape. Then trace along the contour using a fixed-length offset between the curve and the pen/pencil/marker. The curve is thus transferred onto the panel. Cut it out, hold the panel up to the curve, and repeat if needed until you're satisfied with the fit. After one or two iterations the template will fit the curve closely enough that the marker can be slid directly on the surface with no additional spacer/offset. It's OK that the template doesn't cover the entire space that the finished piece should cover -- make a top half and a bottom half separately (for example), then join them together and use the composite template to cut the real material.
Here the cardboard was held up by being screwed to the diagonal 1x2. The straight edge and pencil are shown clamped at the top corner in approximately the position I held them as I slid them together to trace along the roof. I didn't have a helper to hold the camera or the props, so I used a clamp to hold the props..