See if there is a dedicated fiberglass supply company in your area...they ususally sell divinycell...I would use 6mm H80 plain for core material...it comes in 4'X 8' Sheets...I get mine from Merritt Marine Supply in Pompano Florida...You might try http://boatbuildercentral.com/products. Try to get it local if you can or the shipping will eat you alive...The funny thing is the same outfits that usually sell divinycell also sell high grade marine plywood.

Regards,
Bob