The dollar store sells 20"x30"x3/16" foam board which is working nicely for testing. Also picked up some 1" foam board from hd.
I think it was asked before about seamed stainless steel tubing. I can confirm that it works well. I have the seam facing the bottom. Paid about 15 bucks for a ten foot length of 1", .065"wall.
Thanks so much for designing this cnc machine and making it open source. You can tell a lot of thought went into creating this and the build went together very easy.