Thanks for the replies, chaps.
I wasn't meaning the front and rear splash guards - at the rear of the car there is a 3-4" gap between the new tub and the Z3 shell, which will allow a lot of noise, and possibly fumes and weather through as well. It's the area directly behind the B post to the point where the parcel shelf bonds on.
Tribute supply some 'closing panels' to trim and fit but I really don't think they'll do the job of keeping the sound down very well, so wondered what the rest of you have done
The rear splash guards will help keep most of the weather out but the car is going to be very noisy if I don't seal the great big hole up.
...Hope that makes sense!