Follow this guide at your own risk, I'll briefly explain how I made the battery pack but probably look elsewhere to confirm the best practices.

I wanted to have a bigger battery on my Miyoo Mini so

• There would be less battery sag (stop it turning off unexpectedly when doing something intensive at a low battery %)
• The battery % would be more accurate (because of less sag)
• It would last longer
• I wouldn't have to run the battery to such a low voltage all the time and degrade the battery early.

Then I put the feet on the bottom after measuring the tipping point with an early prototype so it would stand on its own.

I made more than 10 prototypes before I dialed in the model so it would click in just right, mine did right off the printer, I'm hoping others might have the same success.

The batteries I picked up were single-cell 2000mah Lithium-ion batteries measuring
50mm x 40mm x 7.9mm, i.e. the same as the included Miyoo mini battery but lacking the plug.

The Aliexpress listing
https://www.aliexpress.com/item/32919454422.html

The cable I picked up was a JST 1.25mm 15cm which I can now confirm was correct.

The Aliexpress listing
https://www.aliexpress.com/item/1005003390942788.html

Making the battery pack

To combine the batteries I...

• measured the voltage of all the batteries to make sure they were equal (if you don't do this you can damage your batteries and maybe even cause a fire)
• slid some heat shrink over the positive and negative wires on the longer side
• intertwined the negative wires
• soldered the negative wires
• slid the heat shrink over the connection
• heated the heat shrink with a lighter till it shrunk over the connection
• intertwined, soldered, slid heat-shrink, and shrunk the positive wires
• I also put a bit of extra electrical tape over one of the solder joins as they lined up so I wanted to add some more mechanical protection if they got squeezed together (probably overkill)
• taped battery pack together

Printing the battery cover

• I printed this on my Creality CR20 Pro which is essentially just an Ender 3 with enclosed electronics so I think all of the tolerances should work well on any Ender 3, your mileage may vary with other printers though I feel like it should be fine.
• print in the orientation shown in the slicer preview, this way the clip will be strong as the layer lines will be continuous around the bend.
• use supports, I had a lot of success with the tree supports as shown