I've described my technique for making styrene roofs before but some of you may appreciate seeing it again.
Generally speaking unless you have a piece of moulded plastic any roof made out of flat sheet forced into a curve is highly likely to sag in the middle where it is unsupported.
The solution we hit on is to start with a flat roof and add shaped ribs which support it along its length.
You can also see in the picture that I have prepared the roof skin - a piece of 15" sheet - which has triangular cut outs where the domed ends will be shaped with the help of Milliput.
What I will do when I come to fit it is attach one side with a very generous dose of solvent and wait for it to set firmly before turning it upside down and rocking it over and bonding the opposite edge, holding it down with very firm pressure for a few minutes until the skin stays stretched in position.
I'll post a post a picture of that when I've done it.