Funny, I called Axiom asking the same question just a day or two ago. Mike or Brent, can't remember who I was talking to, mentioned lead shot or sand would work well.

Since Canada went lead free a couple years ago, I can't think of a better way to use my leftover #4 and #5 lead shot from my shotgun shell reloading days. It wouldn't take much volume to make the base real heavy and stable against tipping.