This is a DIY single 18650 rechargeable battery pack to charge 5V gadgets.
The case cover comes with slide function.
Use "Flexing battery holders 18650" from Heinz Spiess... (thanks for that :-)
Which fits perfectly in my case. Use little hot glue to fix it.

Charging electronic you can easily buy on eBay:
Seach for "5V DC-DC Step Up Li-ion 18650 Battery Charger 134N3P Module Board DIY Powerbank"
May you have to use supports, may not...
Push the electronic in place and fix it also with little hot glue.
I recommend to connect everything with wires and check for proper operation before to use glue :-)
To see LED status (red) i used transparent PLA.

Anycubic I3 Mega

No

Yes

0.2

100

Any material does the trick.

This is designed with Fusion360.
I added the original Fusion file and also STL files for quick printing.
For slicing i use Ultimaker Cura or PrusaSlicer