The green florist foam is good to work with, easy to sand. One tip is when you glue them, do it symmetrically to the bike. That way you can better see if the curve is symmetrical. If you just glue them any which way, the glue lines will throw off things. Easier to do it when the bricks form a sort of grid.
The pink or blue home insulation is good too, harder to sand than the green but good in a way since you can really fine tune the shape better.
What ever you go with test out a scrap piece to make sure the fibreglass resin doesn't eat it. If it does you'll have to cover it in something like plastic tape or tin foil.