Triad Tactical, call Steve Sheldon, order a sandbag from him and you are good to go - 25 bucks but I do not regret paying that since the bags are so well made and handy. Then find a two dollar carabiner and you can hook it to your pack, belt or where-ever. Works slick. I can send you an image if you like.
If you prefer to make one go to a craft store and buy a bag of plastic beads for making beenie babies, sew up a little bag to the size and shape you prefer.