I have a scrap s3 with a Prima in i want to put in my 90, i havent thourght that far ahead as to waterproofing.
A not uncommon transplant in series Landrover's so your not alone.
I'd be interested to hear how your 90 performes with the prima.
There is a guy on the series2club web-site forum who has a website all about makeing a prima fit a S2 & in a great deal of detail i will post the web address when i track it down again.
DM :)