How about this eggcrate stuffDiana wrote:
Then press something into the bag to create score joints where you will later break it apart.
Its actually light diffuser for a suspended ceiling, but the cichlid guys use it under their sand/gravel to prevent the glass cracking.
It looks like the ideal stuff for diana's idea of turning Martin's mix into ice cubes! I'm fairly sure it can be picked up in different sized squares
TC