Go to www.Accudock.com - they have a 67 pound work float 4 X 8 that holds 1000 pounds - and others up to 8 X 40 that are great - see pics
I have used a foam sold in the building trades that will tolerate Polyester resin, it has noticable fiberglass strands in the foam it come with foil surfacing on both sides, I think its sold for flat roof insulation and I think its labeld R- something,,, the last time I bought any it was about $24. a sheet 4'x8' best of luck!