I got tired of working hard to avoid damaging cells as I pried them out of my commercial one cell holder after charging. That thing was sized for unprotected cells, but even those weren't very easy to remove. So I designed this thing in Freecad for charging one 18650 cell at a time, either an unprotected cell, or one with BMS protection installed... of course with ease of use being a priority. It uses 4x "Keystone 209" battery clips from Mouser:

https://www.mouser.com/ProductDetail/Keystone-Electronics/209?qs=sGAEpiMZZMsQtlBhqKq43U9R475cjEbS

I added "+/-" polarity symbols in both trays in case non-colored wires are used. The symbols can optionally be filled in with a contrasting color ABS slurry or paint for better visibility; my uploaded pic shows my results with white abs paste. The side cutouts are sized and filleted to allow easy battery removal with my big fingers. I also added several structural fillets so it will hold up to field use.

I wired my battery clips in parallel with a 18awg fine strand silicone insulated wire, and terminated them with a Dean's Ultra connector to work with my R/C hobby chargers. The 4mm diameter wire holes were a perfect fit for that wire. Of course other types of wire will work, but if your insulation OD is >4mm you may have to drill the holes larger.

I printed in ABS, because mine will end up in the trunk of a car during a hot summer. Otherwise any filament should suit the purpose structurally. I designed this thing to print with 2 walls at 0.45mm line width, and 0.2mm resolution. Other settings should work, however I recommend sticking with walls >=0.9mm thick to ensure the battery clip walls provide adequate support.