Silica sand - THE END. Get yourself some through a janitorial firm (silica sand was always used for public high volume ashtrays). Noel is a genuine guy and his product typifies beauty in simplicity.
Coincidentally, I just the other day listed one of Skylan's audio racks for sale on a couple of different (free) sites, and have been absolutely inundated by interest. I own 2 rack models and 2 sets of speaker stands, one pair of which were configured in the mid-80's, and just so good, I've never been able to let them go - despite owning only planars for the past 6 years. (you never know when I might need those stands again!). They're a load to move around because they're full of silica sand - solid and dead as can be - not sure why you'd use anything else. Hats off to Noel and his excellent, simple products!