1st of all I want to give credit to enif (remix link) for solving the dreaded contact solution for battery boxes. The post for wrapping wire around is a great solution.
I was trying to optimize a battery holder for hobbies. I have a few early versions but they were all to tight. I decided to make use of my printer's inability to fill 1mm walls. This makes for a perfect spring at both ends of this Li-Ion cell protector. You will find this device to have good range for battery sizes. However, it is targeted for protected cells which are typically 2+mm longer. The design distance between contacts is 68mm.
Also notable about this design is that as you expand the ends, the sides pull in. Sort of like a finger puzzle. Although the overall structure is pliable, a battery will have a hard time being knocked out of the the frame.
The idea here is to make your protected 18650 Li-Ion cells safer to carry. This little frame will let you dump your spare cells in your backpack with little worry of shorts. Consider it a backup protection to the cells internal protection.
But some of you will see the use in making this a battery holder with leads. I find wrapping the wire tedious (22awg solid) so I got out some copper foil. As you can see in the images, this is a very easy implementation. Copper, brass, and stainless steel foils are available at your local hobby shop.
And of course, this is only the main structure. I'd love to see some remixes where multiple boxes are ganged together and a base is added. For now, I'll leave this for you good people to play with. And I will get on with the project these are intended for.
Tech details: The active spring region is 1mm thick; prefer no fill. The typical wall is 1.8mm to ensure 2-trace wall thickness (2 in + 2 out=4 total). The upper and lower cell-retaining ridges are different so there is a top and bottom. Normally, it won't matter but as you fix this to other things, it may. I recommend you print this with a minimum of 4 walls.
As to material, anything should work... I recently began printing with PETG. That works well for this. But please don't use any of the exotic conductive materials. They will drain your batteries before you ever get to use them.
Edit: 12/6/18 - I've added the 2x 18650 cell protector. This one too converts easily into a battery holder.