what type of sand to use?

Children’s sandbox sand.
It is guaranteed to have no foreign matter.

My favorite thread -- it's the only one I can answer with genuine authority, haha. See my now aging response here:

http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1043108105&openflup&7&4#7

Sand type is not so important as making sure the sand is cryo'd first

What are the risks to the planet if you mix Alien Sand with Machina Dynamica Brilliant pebbles?

No,no,no.Do not mix them.That stuff is quacky.My Alien Sand is the real deal.Beware of false prophets.Stick with proven,real life acessories.Only 5 pounds left,act now.

Play sand mixed with small gauge lead shot should do the trick nicely. The differing density of the two materials mixed together deaden the sonic signature of the rack without emphasizing or de-emphasizing any part of the frequency band thus not allowing the stand to have a sonic signature of its own.