I don't know what kind of back seat, if any, the roadster has...

It's got a 'ledge' which could seat a small child or a dog comfortabley, but the rest of us use it for shopping. You could get a decent size sub enclosure up on there, but when the hood's down it fills the space.

However, I was thinking more like actually on the floor behind the seats - I have my seat all the way forward, and since MGB's have amazing legroom for a sportscar (more legroom than most modern non-sports cars, in fact), the passenger seat could stand to shift forward a touch in order to accomodate enclosures.

On a different note, have you seen these?... http://www.elementenclosures.com