urethane foam
 

Looking around for a cheapo core for my T top roof,
I tried some 3/4 inch insulation urethane foam from home depot,
peel the silver backing off one side,
slit it into 1 inch squares w/ utility knife and worked polyester putty into it,
then applied resin and mat, let it set and its rock solid.
No reaction with the polyester resin at all.
Only $12 for a 4X8 sheet.

Not sure I'd be doing a deck with it but for various other projects its fine.

Just passing on a savings tip.