The Lithium battery pack module prototype is achieved!
The prototype was adapted with cutting tools and 3D model modified for a future print so that all will fit welll with no need of adaptation. This prototype is a tool to calibrate a finalized model, but works fine. Switch is on the side, so that the casing closes.
It fits a 3DS XL casing with the Gateway traveler and the 3DS XL casing can close with the 3DS XL inside! If you plan to put it in a 3DS classic casing, it will not fit, unless you use only the 3xAAA battery pack, suited either for a 3DS casing or a 3DS XL casing.
Here pictures of the lithium battery pack module: (click to enlarge)
Recharging lithium/ion module and charging LED indicator:
(I powered on the lithium battery module so that we can see that charging voltage is almost 4.2 volts = max charging voltage, but with a secure voltage little drop)
Video of use and demonstration of fitting a 3DS XL casing:
Here pictures of 3D model of printed elements to assemble for lithium battery pack module: (click to enlarge)