Place a big zip-lock bag inside of a shot bag, and fill with sand.
Roll/squeeze all of the air out of the zip-lock bag and seal.
Fold over the end of the shotbag and sew closed w/ nylon thread or monofiliment fishing line.
Mine worked very well for many years, until I loaned them to the son of a sportsman's club member.
He never returned them, or the .50 caliber can they were stored in.
Now, I use an Uncle Bud's "Bulls Bag" filled with sand.