"a roadster"... as in, a z3? If so, should be able to buy all the clips and fixings for the hard top which they made for the car. Shouldnt be too hard to make a thin metal frame which connects all the bits, and then cardboard, parcel tape and grp to connect them. Same with an MX5, and with the price of those hard tops, I wouldnt be surprised if someone has done that a few times already.
I wondered if something like the lightyear fastback top might be a good starting point, if you can find one (or something like it) on this side of the world (light-year are aussie-based).