How to Make Medicine Balls
Much of this information is from a post that originally appeared on Submissionfighting.com. The post was written by Grandpab. I highly recommend you check out the forum. I reprinted, a somewhat edited version of, the post here.
The first thing to do is to get some basketballs. I think that smaller basketballs and volleyballs would be useful, also. I already had lighter medicine balls, so only used full-size basketballs. Both of mine were obtained from the Goodwill, for around one dollar each. They would likely not hold air, but that isn't important for making medicine balls.
First I cut a three sided square, about two inches per side, in the basketballs. It is useful to leave one side attached to the ball. I then filled the heavy one with sand and the lighter one with sand and cloth. They weigh around 22 and 26 pounds (Note: this is an estimation. Both of my scales are inaccurate.). At first I added water to the heavier one. This added some weight, but the seal eventually broke and it all leaked out. I wouldn't recommend adding the water. The additional weight was minimal, and it was messy when it leaked out. For an even heavier ball, it is possible to add some lead shot. I haven't tried this, yet. After filling, I sealed the three sided square with shoe glue. I then added several layers of duct tape. The balls have held up very well.
There is also a article in an older issue of Strength and Conditioning that talks about making medicine balls. In the article they use cheap playground balls and add water, with a air pump, to them. It might be worth trying, if you are looking for some lighter balls.
To learn how to make a medicine ball rebounder, click here.
REVISED
I recommend using a LOT more duct tape than is shown in the above photos. For duct tape I recommend 3M Highland 6969 duct tape. It isn't cheap, but it is 10X better than normal duct tape.
I have since made a 60 pound monster. It is a basketball filled with lead tire weights and a little sand. Used lead tire weights are available free or at low cost at tire stores. I'm not sure if it will hold up long, but so far so good.
I also plan to use pea gravel instead of sand in a lot of applications. I doesn't leak as easy and is easier to pick up. In an apartment, sand seems to get everywhere.
I have discovered the balls hold up better if they are 100 percent full. Try using stuff that will give you the approximate weight. I plan to make some with flour and rice, much like my Clubbell© substitutes. This is REALLY important, less then full one seem to break a lot quicker.
Another trick I learned, I think at Crossfit, is to patch the hole. Use a large patch made out of an inner tube or piece of rubber. Glue it on with patch glue or shoe glue.
Happy throwing!