For what it's worth, I filled them each with 25.57 pounds of plain kitty litter to fill each half way to the top. Filling them half was with kitty litter was recommended by David Fabrikant in e-mail, and it worked out perfectly. I would not want them any heavier and the kitty litter dampens them very well.
Plain kitty litter is just bentonite, a naturally occurring clay mineral. I got Dr. Elsey’s Premium Clumping Cat Litter because it is pure bentonite: no scent or extra chemicals (as far as I can tell). I used a shipping scale and did the filling in my kitchen. There was very little dust (I hardly noticed any).
https://www.amazon.com/dp/B0009X29WK
I am talking to myself here, but hopefully it helps someone down the road.