Sand Sock?

Sniper tool. Army wool sock 2/3-3/4 filled with sand. Fill yours to fit. I am not a real sand fan, as it filters thru the bag and scratches up the stock/action bottom. But you can also use corn cob media to fill the sock. The corn cob is not as hard as sand, and can tend to shift worse. Other option is to put the sand in a freezer bag inside the sock. Then no leakage AND it forms up hard, unlike the cob media.

Best news is, with a sock, you can keep adjusting the fill 'til you get it just right. I would recommend using a zip tie to seal the sock for very precise bag size. Hope this helps.