Karen,
Congrats on the purchase and welcomed me to the IP family! Although I have not had to replace the mast boot on my IP37, I do know this topic gets discussed on here periodically.
If you check out the link below, Hayden has documented how he maintains the mast boot/ deck joint on his IP35 and also outlines the material he uses.
Regards
Pete
www.ipyoa.com/forum?view=topic&catid=46&id=2873#2873