29mm is as small as you want to go, most consider this size perfect. But the 31mm that I linked works great there’s a little more room to work with and does shrink down tight to the battery even at the cut ends. I did 2 batteries just a few days ago, works like a charm.
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Edit: By the way that is flat heat shrink, measured across the tubing when flat, like in the pics a ebay.
Good thread that might help. Tutorial: Rewrapping a battery