I’ve always heard kiln dried sand. Many use Cat Litter, though I’ve always heard it best to use plain. You want something inert, dry, non corrosive... if it wasn’t toxic, lead pellets are best. Again, anything like those mentioned above that will keep steel from rusting or corroding.
for the feet, if it doesn’t add much cost, you could always use an M8 female screw on bottom of stand so youhave the option of choosing spikes or feet if your situation changes. (Spikes are better for carpet.)