Has anyone used those for the cake board? (the bottom one that you usually decorate to match the cake)

ETA: Let me clarify, the boards that are near the paper/project sections, I would have used it in school for project displays)

I have read that some have double them up with two layers and used them for cakes that are not too heavy.

I use 1 layer of foamcore board that is 1/2 inch thick. I love it for my tiered cakes, because when you use the wooden dowels for your support system with a long center dowel through the whole cake, you can drive the dowels right into the foam core board, and that sucker won't move at all.
I have found that here in Upstate NY foamcore sells for $5.99 at michaels ; a little more at ACMoore, and its cheapest at Hobby Lobby at $4.99, but I find that Hobby Lobby also has a 1/2 price sale on it every couple of weeks, so thats when I stock up on it!

I use it all the time. I get it at michaels about once a month when they put it on sale . I use the 3/16 and the 1/2 ". Love it

I use it all the time. I get it at michaels about once a month when they put it on sale . I use the 3/16 and the 1/2 ". Love it