Originally Posted by bri1977

Foam board (sometimes called foam core) is more like styrofoam with a coated surface. It is used for flush mounting graphic prints and such, so I would imagine you could get it at any store that does framing, but I'm not 100% sure. The cabinet came with a wooden piece that was not fitted right, but there are little "tabs" for it to rest on. It was 3/4 inch thick so I glued a 1/2 and a 1/4 inch piece of foam board on top of each other with spray adhesive to make it the correct thickness. Before I glued I measured the opening and cut it (making sure it was big enough to fit tight) Then, I placed my machine on top of the boards and traced it. I cut it out, again, making sure it would be a tight fit. I cut the back side out completely so I could slide it on from the front and be able to reach my hand in to turn it on. I can't access the feed dogs switch unless I remove the board, but I could always just cover them to quilt. I used a utility knife to do this. It was tricky. I am very skilled with the knife though, because of my graphic design background. I cannot tell you how many times I have cut foam board and matte board so BE CAREFUL if you attempt to do this.
Thanks everyone for the nice comments :-)