Any dense foam or polystyrene will do - closed cell EVA is the best, but it is expensive - what it does is allow the glass to expand, contract, and warp (if you only heat one side, like the inside, glass tries to curve) without causing 'stress risers' - points at which the glass gets adhered to the stand causing an internal stress, which can become a crack - its the same principle as mounting glass in rubber, rather than directly into the aluminium frame.