I have few devices that use 21700 batteries, but i only have one. I have ton of 18650 batteries with higher capacity so i thought about using them instead.
There are two models, adapter and sleeve. If your device has longer spring (since difference between those two batteries is 5mm) you can print sleeve which is only 65mm long (same as 18650 battery). Adapter is 68mm long, but you need to add M6 nut, which will bridge that 5mm gap so total height will be 70mm. You will need to push it in with soldering iron or heat it up some different way. You can also put a little bit of superglue, but mine is holding in pretty well on its own. Tested with few devices and it works just fine. Alternatively you can add bolt inside of nut if you need a bit more height (button top battery). You will need to cut bolt to correct dimension.
It's a bit tight fit, but it fit all my 18650 batteries. Let me know if i need to correct dimensions.
The author marked this model as their own original creation.