The inspiration for this came from a video I saw for grip strength work on My Mad Methods. In the video the guy had a loosely filled sandbag that he threw from hand to hand. The sand was constantly shifting so the area to grip was constantly changing. This unpredictability and fluidity is perfect for natural training.
The bag I made is built much like a Bulgarian training bag.
Take an old tire inner tube and cut out a section (300mm or 12 inches would be a good start). Fold over and zip tie one end. Fill with sand. I wanted mine to be 8Kg which I thought would be a good weight to be throwing and catching.
Once the bag is the desired weight, lay it down on the ground and flatten it out. You want to be able to get your hand around it in a flat pinch grip, but not too flat, remember the sand is going to move around a lot. When you’re happy with the shape, tie off the other end.
Test it out, if it’s not right cut the tie off and put a new on one higher or lower. When you’re happy, cut the excess rubber off, and duct tape the short end pieces up.
You now have a throwing bag ready to use. Throw it from hand to hand; try some kettlebell movements like alternating one arm swings and juggling. Have fun and feel the burn in your forearms.