I use the tubing to label cartos sometimes. I cut a small ring of ⅜ tubing of whichever color and shrink it around the middle.
If you want to find the tubing real cheap go to a computer parts store. I get mine at my local Altex for around $1.50 for a 4 foot length. Granted, I use the stuff for other computer/electrical tinkering, so I have many different color/sizes laying around already. Also, having a variable temperature heat gun makes it much easier. I takes seconds to shrink as apposed to minutes.
As a warning, I would say to be very careful doing this with
batteries. With heat guns you are getting anywhere from 250 to 1000 degree air. You want to use the lowest setting and direct the air to blow on just the edge of the tubing and not at the battery itself. Also remember the tubing will act as an insulator, so a battery used heavily will hold the heat longer and could increase the chance of critical failure.