Not sure what constitutes expensive in terms of foam, but I picked up a pack of florist cubes last night for about $6 USD at Michael's. It looks like plenty to shape a bump stop, probably around 18"X6"X12", maybe a little more in four total pieces. A can of Great Stuff runs about the same at the Home Despot. I imagine florists can get more for the foam, so check a craft store if you haven't already.