I usually pick up the Harbor Freight brand foam mats which are 1/2' (12mm) thick. It's quite a bit thick, but it makes it easier to put relief cuts in the back for folding and bending and adds a lot to the over-all rigidness of the piece. Then use the smaller mm foam for details like Colbs said. A hot knife will cut throught the foam like......well a hot knife through butter lol. Also, a sharp pair of scissors can also cut through it very well.