Battery holder (UM 2X4) for charging 18650 batteries

If you have, like me, a bunch of these holders and don't know what to do with them, you could repurpose them to charge 18650 cells. These holders are made to hold 2x2 batteries of type C or LR14. But more importantly, the size of one LR14 is about 26.2 x 50 mm. No surprises here, you can fit two 18650 in one holder and charge them in series.
If you have a 3d printer, things get simple. You print 2x2 rings, first ring with Dext=26mm., Dint=19mm., L=30mm. and second ring with Dext=26, Dint=10.6 and L=25. Different rings sizes will work; up to you to decide.
You then take an aluminium rod (d=10mm) and saw 2 pieces at L=30. You can use other materials. This is just what I had available. Copper or gold :-) should work just fine.
If you use anodized aluminium, you must file the ends because it's not conductive.

When connecting the cells to your charger, make sure not to short them!

Note: you do that project at your own risks!