There are a few older threads talking about Fenix ALF-18 18650 to 21700 Battery Adapter but its to pricey for me. Are there any cheaper alternatives or maybe some 3d printer design files I can use to diy? Wrapping seems easy enough for the diameter 18mm to 21mm. What about the height differences 65mm to 70mm and conductive material through the bottom for ground?
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I think it’s better to bought a proper one from the same brand, isn’t that expensive… (there are some others with double the price, even more) !
It is a bit pricey. You can get a good 21700 cell for the price.
Depending on the light, you may just be able to get a couple of O-Rings of the right size. It depends on the springs in the light, I guess.
I have so many damn batteries, I haven’t looked at adapters. Actually I have 6 or 8 adapters that came with different lights. They are sitting in a drawer and have never been used…
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Take your electrical tape and wrap 3 full circles around 18650 bottom and top part of the cell.
Some aluminum foil works in a pinch if the battery height difference is off.
Many lights will take an 18650 without needing to extend the length, and it’s preferable that you don’t unless absolutely necessary (something like that Fenix adapter being a better option than foil, especially if the light draws much current). There are loads of cheap plastic open-end sleeves to take up the slack and the better of these have internal ribs so as to allow a little air space (air in this case being preferable to heat-insulating solid plastic, but both will increase cell temps since far less can be transferred to host metal). Sofirn includes them with many lights and I think they still list them as a separate accessory on aliexpress (or you can message them and they’ll sell it). You can find them all over aliexpress and there are some on amazon as well (and ebay). Brightguy, Killzone, Battery Junction, etc might also sell the cheapies.
That Fenix is on sale for $6 on their website (plus shipping…). There’s a similar one on amazon now for $10 shipped from china. Amazon.com
These are slightly tapered so they’re a tight fit in some tubes (which is fine but they’ll take more effort to remove, or you could use a fine slitting saw in a dremel to slit the tube and help with that): https://www.amazon.com/LampVPath-Protective-Converter-Batteries-included/dp/B01MY2ZSYT
O-rings are the next best option, still allows some air space unlike wrapping with paper or whatever. If you really need to add some length, you might consider the little button-boards that Simon sells and put one on the + end, adds about 3mm (could use two if necessary…they center well in the tubes). https://www.aliexpress.us/item/3256803132204249.html
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It depends from the flashlight construction, it might need a proper top on the battery, to make a good connection… !? Go ahead and bought the proper adapter from the same brand… ! Or even better, make some savings for a while and bought the proper battery… ! It’s gonna be much cheaper on the and of the day… ! Just saying… !?
Yes… but if one is in need of light, and has some charged 18650s, but not 21700s… Well an adapter of sorts will be of value. This is the reason that I have so many of different types in a drawer waiting.
Or maybe I am just a fanatic/pack rat… 
But yeah, for regular use, the proper cell is the way to go.
Guys please, be more careful with your equipment and how you use it… and for your advices to other members… especially when you don’t know his level of his experience… ! Im trying to say - “play the smart guy”… but it doesn’t take a lot to have some bad situations… … … ! Be careful please… thats all… !
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Not a terribly hard thing to make . I have a couple that came with the firefly lights , kind of clever and easy enough to make yourself .If you have one of those sleeves all you need to do is make a plug and something to make contact like a nut and a bolt , a little glue and you’re all set .