My wife wanted a 2.5 lb. and 5 lb. medicine balls for her light workouts, but couldn't find what she needed at any of the sporting goods stores. I figured I could make the size medicine balls she wanted for next to nothing. All I needed was a child's ball ($7.99 x2), a funnel (I bough an automotive funnel ($1.97) but found a smaller, better one in our kitchen), playground sand (I had a bag in the garage) a scale (I started off with a bathroom scale but ended up using a kitchen scale) a tube of clear bathroom/kitchen caulk ($3.59), a pocket knife, and a cookie sheet.
Step 1: Preparing the Ball
I used my pocket knife to cut a small X (slightly smaller than the end of the funnel) at the top of the ball (photo disappeared). I made the first cut with the small blade of the knife but then finished the X cuts with the scissors. I then inserted the funnel in preparation for the sand.
Step 2: Filling the Ball
My bag of play sand was damp from being stored in the garage, so I spread it out on a cookie pan and baked it in the oven for 10 minutes at 400 degrees. When it was dry and cool, I used a measuring cup to pour it into the ball through the funnel.
Step 3: Weighing the Ball
At first I was using a bathroom scale, but didn't feel it was very accurate – I had to weigh myself, then weigh myself holding the ball. So I found my wife's kitchen scale which was the perfect size and more accurate.
Step 4: Sealing the Ball
When the ball was at the desired weight (remember to compensate for the removal of the funnel), I inserted the nozzle of the caulk tube and spread caulk around the inside edge of the hole. With the nozzle still inserted, I turned the ball upside down so the sand covered the hole and the still inserted caulking tube nozzle, and squeezed in another dab of caulk inside the ball. I then removed the nozzle, wiped away any residual caulk/sand and added a dab of caulk to the outside of the hole.
Step 5: Finishing the Ball
With a slightly wet finger, I smoothed the dab of caulk to form a slight mound making sure the edges had good contact to the ball's surface. I set the balls aside for 24 hours to dry, but suggested my wife to wait 48 hours to insure the caulk inside the ball had time to completely set up.