Here's the base piece... this part is still a work in progress:

https://cad.onshape.com/documents/16f7d96e75fbf5ac5831daa7/w/67b7b3a8fe5e4762c436ad20/e/7f3751909f21b9045d0211f7

The intention is to have it 3D printed, so I can include slots to hold battery terminals for the 18650 battery (or stack of 2)

This is the negative terminal I'm probably going to use:
https://cad.onshape.com/documents/c25da064e829ff49b4182c16/w/1f7ff295f5cdafe13c14b472/e/854bc82a685523a38a3620c1

More as time permits...