I have read concrete blocks work very well.Find some a foot to 18inches depending on how high you want them the use liquid nails to glue them together.Paint them then get some blue tac or equally good putty at a hardware store. make 3 or 4 pea sized balls and put them on top of the block then put your speaker on and make sure they are where you want them then push down hard on the speakers but make sure the speakers arent touching the blocks Dave
