Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4946485.

Wii U battery door with space for 2x 18650 and TP4056

v2: Added a more protected battery tray with spring contacts. Also added a switch to turn off battery.

Battery contacts: https://www.amazon.com/gp/product/B01DZ8AYX6/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B01DZ8AYX6&linkCode=as2&tag=printcrab-20&linkId=b784e9093a724d931d082fe18ead06d4

TP4056: https://www.amazon.com/gp/product/B01DZ8AYX6/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B01DZ8AYX6&linkCode=as2&tag=printcrab-20&linkId=b784e9093a724d931d082fe18ead06d4

Available on Etsy:

https://www.etsy.com/printCrab/listing/1076893524/wii-u-gamepad-extended-battery-usb-type?utm_source=Copy&utm_medium=ListingManager&utm_campaign=Share&utm_term=so.lmsm&share_time=1633129810952