The entire body of each cell excluding the positive button, is electrically connected to the negative electrode.
That means any contact between the positive-side nickel and the cell body is equivalent to a direct positive-negative short circuit, AKA a Very Bad Thing.
Anywhere you have a conductor that isn’t connected to the negative, anywhere near, touching, almost-touching or could-touch, you need something to insulate between them and prevent shorts.
The cell wrapper and “regular heat shrink” aren’t heat- or abrasion-resistant enough for that job alone.
You need either fish paper, or polyimide sheet, or some other tough, heat and abrasion resistant material.
Kook is a bit of an abrasive shithead, but his advice is solid. Treat your battery-building as if you were working with fireworks or other incendiary devices, because that’s what they become if you make one wrong move. A bucket of sand is a reasonable precaution to take.
This, plus this thread as well. Battery Basics for Beginners Wiki [Serious] We try very hard to impress upon new builders just how unforgiving battery building can be.
It’s not at the level of high voltage/high energy physics, where you’re basically poking an Old Testament God in a can (Lots of rules, zero forgiveness), but it’s not too far removed from that.