I have the same issue sourcing batteries. You can find some on eBay but so pricey you can often get another used shuffle. And other sources for lipo batteries are often offering 150 mAh as the smallest capacity which is still way too big (original is 50 mAh).
Also I noticed the original has a protective circuitry built in. So just any lipo won’t do unless you transplant that very small board. Normally lipo’s rely on their chargers to be balanced and protected, hence having balancing leads (for multi cell configurations).
I though I had a good idea to buy this very cheap mp3 player (for < 3 EUR) and extract the battery from it but even that battery is way too big. I’ll keep looking in that direction though. Other electronics with very small batteries. https://www.benl.ebay.be/itm/Mp3-Player-...
Check my comment in the article on that specific step. You don’t have to drill out anything. You can use the clip on the shuffle itself to slightly force the back plate to bend giving enough room for the clips to snap off.
Sure that’s a good plan also. Just make sure not to short the wires in any way and detach the negative wire first on disconnect & negative last on connecting again.
For a connector you’ll really need a very small connection then cause the space is very limited inside.
I managed to open mine using no tools at all so without damage. Maybe the glue also weakened over time so might need to use some heat. I didn’t but then again it’s a second hand, old device in my case.
I used the clip on the back as a leverage to (ever so slightly) bend the back piece in the center. Having that bend is enough for the very small clips on the power switch side to detach. Once that side is removed you can slide the complete back out towards the direction of the power switch. I checked on a flat surface and the piece isn’t damaged, still straight. Just don’t keep it bend too long and don’t over bend it. It really shouldn’t take much since those notches are really small.
To close it again just slide the back side in again and then snap the power switch side back in place by lightly pressing it on top. Since the clips aren’t damaged this way it will close up nicely. If the notches got damaged you’ll have to use glue instead (hot glue will do and is easily removable again).