Dirk,
use the blue foam you're talking about. its cheap and VERY rigid. It is every bit as rigid as the poly cores. to shape use adhesive backed 36 grit abrasive on a flat "stick" as a file. then sand with 80 grit before glassing. you will HAVE to use epoxy. everyone is right in saying that polyesters and vinylesters will melt the blue foam. I recommend Gougeon bros Pro set. stay away from the west systems. west is made for coating with light amine blush, but is not structural in anyway. in temps above 80 degs it will become soft and heat deflect. I learned this the hard way with bicycle frames. we shaped blue foam cores and wrapped carbon with west systems. I always thought it was my imagination that the bike was more flexible in hot weather until i made some flat carbon landing gear for my rc plane and watched the wheels turn outward 45 degrees in mid 80's weather. Anyway, shape away and have fun, would love to see pics of your progress