Zerogee
Clencher's Bogleman
For a gently curved roof such as the one needed for the tram build, check out what GRS (Garden Railway Specialists) offers in the way of pre-formed curved styrene roofing for rolling stock scratchbuilds - I have a length of coach or van roof that I bought there several years ago, about 40cm long, which would probably be very close to what you want for this tram....
Jon.
Jon.