im 'this' close to putting my rl-p back into i.b. config-the sound was just to my likeing- but it will mean sealing up the trunk (fold down seats-rear shelf) properly. im thinking fibre glass and mdf and bondo and expanding foam? any sujjestions before i make mistakes?

Take your time. There will be spots you will miss.

Depending on how extensive you want to get will be the determining factor. I spent over a year on my last project just sealing the trunk and cabin off. This included cutting and rewelding steel so I could get access to some spots.