use balsa filler mixed with some epoxy, works great. a little tuff to sand, so be sure to appliy it even and smooth and wipe of any excess

If you just need a little, your better off with red devil spackle compound, it has no weight and sands easy, if you need a lot, glue in another piece of foam and sand it smooth. Any glue you use is stronger than the foam. You wan to keep from making it heavy.

i only patch little spots and that is what i use, it works perfect for me

I made some nice fillets on a foamie by mixing Red Devil LW Spackling compound with epoxy. Worked great and was very lightweight.