Hi,
Have a look at this:
http://www.just-carcovers.co.uk/waterproof-half-car-cover-bmw-z4-i1766.html
I bought one last year, and has been rather quite useful to avoid iced windows stopping access to the car! (glass not dropping so door glass wedged behind seal)
I've got a coupe, and it falls just short on the rear windscreen (about 3-4 inch line where it meets the boot lid - I think would be ideal on the roadster, and fully cover the roof quite easily.)
Fits snuggly when both wing mirrors are in the pouches to minimise wind etc.
This is a good quality product, not a cheap sheet of plastic!