I built some stands in the first part of this year, as they are steel I was concerned with moisture embedding in kitty litter as it is hygroscopic. Steel and aluminum are both susceptible to corrosion/oxidation. I recall deciding that if budget is no concern, copper pellets or BBs would have been best. Steel shot that I have encountered, already has evidence of oxidation/rust so I would not introduce that into the stands. I spent an inordinate amount of time researching material and I decided on quartzite. If I recall correctly it is heavier than silica sand, it naturally breaks to a rough edge and therefor locks together really well once inside the stand columns. There is a company that produces crushed quartzite and it is labeled as Cherry Stone. I purchased a 50lb bag from Rural King for $7.99. It has worked out really well for me.
The filling options are endless, sand blasting media, epoxy sealed aquarium gravel, or as mentioned above, glass beads.
I would avoid kitty litter for sure.