A trick used in some boats is to mix lead shot in resin. Basically you add a little resin to the shot and roll the shot around and around until coated in the resin, and all resin used. A lot of shot will still be uncoated and loose, and so add a little more resin and repeat. Eventually the resin and shot will be evenly distributed, with the maximum shot to resin possible.
This can then be poured into the hull (useful for deep shallow keels, or keel bulbs), or put in poly bags and shaped into the hull and allowed to set. The poly bags stop it sticking to the hull.
Not sure if you could do that with your sand.