One further addendum to the PlaySand suggestion - You'll want to make sure that the sand itself is moisture-free (and I don't just mean cat piss). Sometimes the sand comes with moisture included at no cost to you, until you fill your steel stands with it and the moisture causes the insides to rust. I've heard of folks pouring it into a few big pots and sticking it in their oven at low-temp for several hours to bake off any moisture in it. Sometimes you'll just luck out and find a source that sells bone-dry play sand and as long as you keep it from your cat you're good to go and won't need to Bake any sand pies. The alternatives are all more expensive, short of kitty litter which is not quite as dense or heavy as sand. Lead shot is available from gun dealers who sell reloading supplies, but you'll have those tiny balls rolling around your floor for the rest of eternity. They can be a bit annoying when you step on one. Also lead is not the best thing to be handling, or for pets or children to have access to. I don't know if they make shot out of lead anymore though. Anyway, that method definitely will add more weight than sand. More expensive still is metallic powders marketed by the likes of StarSound (who make excellent isolation products, btw), which will add more weight still, but will take weight from your wallet. Pick your poison.