I use a "bat weight" such as this: http://www.amazon.com/DeMarini-Bat-Weight-Red-16-Ounce/dp/B005EZ3ABW/ref=sr_1_2?s=sporting-goods&ie=UTF8&qid=1463515414&sr=1-2&keywords=BAT+WEIGHT
I attached two rare-earth magnets with electrical tape, and voila! You simply slide it up and down the bottom of the tube to adjust balance.
I was looking for these a while back. Found the simplest solution. 20 oz bat weights fit barbells.
http://www.amazon.com/DeMarini-Bat-Weight-Red-16-Ounce/dp/B005EZ3ABW/ref=sr_1_1