I ordered a bunch of battery clips from aliexpress (link below) that are pretty solid and cheap and I made some battery holders for it. I think the contacts will be fine for currents of 1-2A at least. For plain 18650's without a contact hump on the plus side like on the photo, add some solder on top of the contact to make it stand out a little more. Freecad source file is attached as well.
2024-07: Improved version with reinforced sides. Added 4x AA battery version.
The author marked this model as their own original creation.