Ask for Dow Styrofoam XPS PIB at your local industrial insulation supplier. Or call your local taxidermist and ask where he gets his foam. I got nine foot long FB blocks that were 10”x20” from a local insulation supplier several years ago. YMMV. http://www.polyguardproducts.com/wp-content/uploads/2015/07/XPS-Data.pdf

Do not laminate or assemble smaller blocks in order to make a block that meets your needs. The others on here are correct: it will result in structural deficiency and likely death. The foam is not “just” in compression as erroneously believed by some; it acts as a very thick shear web (made of very weak material, hence the need for its thickness) between the spar caps. If there’s a glue joint in that shear web, various factors conspire to promote eventual delamination along that glue line leading to structural failure of the wing.

David J. Gall

BSAE