In the sand box

Some use Organics such as Rice as the Particulate, others use Metal such as Lead Shot, other use Grain that is a Stone Particulate, in general all must be thoroughly dry before being put to use.

There is also Oil Compacted Sand worthwhile looking into as the Particulate.

There is a thread started on 3-27-2020 by @soma70 entitled "Sandbox Design Advice". In it I recommended silica sand, a non-organic, man-made material used in sandblasting. Silica sand is superior to play sand for this application in that it doesn’t absorb moisture, and is therefore incapable of supporting the growth of bacteria.

Silica comes in different "grades", just like lead shot. When I was using the Bright Star Big Rock isolation boxes, I settled on No. 60 grade sand. It is the optimum size, anything smaller (the particle size, a larger number connoting a smaller size grain) being too powdery.

I found Silica at a construction supply yard, but I think even Home Depot sells it. It’s cheap, like under ten bucks for a fifty pound bag (this was twenty years ago).

Years ago there was a product that was a sand box with a shelf fitted on top of the sand (I think that was made by Bright Star, as bdp 24 mentions).  I used a pair under my Jadis amplifiers, they were effective at isolating the amps from floor vibrations.  I used play sand.  Seems to me you could make one pretty easily and inexpensively too.  Bdp24's advice is sound, wish I had used silica sand, as the play sand did get a little damp. 

I'd avoid quicksand at any cost.  A few years ago my DAC was swallowed whole, never to be seen again.  Tragically, this incident occurred in clear view of my grandkids.  Counseling can get very expensive.

Grand sand animates the playing of the grand for less than a grand paid by an aging grand.