Ishi sashi, or stone padlocks, represent one of the main pieces of traditional supplemental training (hojo undo) equipment used in Okinawan karate. Similar weights have become quite popular in modern gyms in the form of kettlebells.
If you don’t want to purchase pre-made ishi sashi or kettlebells (they can be fairly expensive) or you find it too daunting to make ishi sashi out of stone or concrete, you can try this creative solution developed by Paul Chandler. All you need is some wood, a brick, duct tape, and a little DIY skill. Depending on the size of the brick, these ishi sashi will probably weigh somewhere between 5-10 pounds.
Paul has kindly prepared some very helpful instructions and plans, which you can download as a PDF file.