What to fill Skylan stands with for Harbeth SHL5s

Skylan stands for SHL5s: could someone please tell me what to fill them with?
Skylan advised PURINA MAXX kitty litter, but I cannot find this in the U.S.
Should I use play sand, sand kitty litter, clay kitty litter, something else?
Thank you.
I placed the above comments as a review so I can add to it later and maybe answer some questions. The review should show up soon, I expect.
I custom made a stereo rack to match the Sound Anchor stands that my Spendor SP100's sit on. I filled 2x4,2x2,1x1 steel tubing with silica sand. Silica sand was plenty dense, heavy and dampened steel perfectly. Steel likes to ring.
Any clay kitty litter will do fine. I just got a pair of Skylan 4 post stands for my Monitor 30's, filled them about 2/3 full, 5 pounds per column, with unscented store brand kitty litter. I've been using kitty litter in metal stands for years. It was recommended to me by a guy who has been a high end dealer for over 30 years. At first, I was surprised that Noel recommended it, since Skylan stands don't ring the way metal stands do and it seemed to me that the only reason to fill the columns is to add weight and mass. But the more I thought about it, the more it made sense.

Anyway, the Skylan stsnds are terrific. Big improvement in bass definition, clarity, soundstaging, depth and imaging. It's like the speakers are suspended in air, but are absolutely stable. Terrific jump factor (which I noticed when my dog jumped).
Skylan stands are awesome.

Noel is the nicest guy and really knows his stuff about good sound.

He taught me more in a half hour that I did not know for years !