Does anyone know where to get a 21700 to 26800 battery adapter? I have a nice one that came with my Lumintop D3 and would like a couple more. I tried contacting Lumintop on their website and their AliExpress store to see if it can be purchased separately but got nowhere. They kept pointing me to unrelated products. Language barrier I guess. I know people are doing it with these O-rings and these Battery Spacers but I'm not comfortable with loose pieces in with lion batteries.
Each adapter is going to rely on the threads of your host light. However we got lots of 26800 around here. 26800 Lights and extras. FREEME ✌ ASTROLUX 26800 Battery Tube for FT / EA Series Group Buy - ENDED Grizzly's Xtar SC1 Plus Review – A 26800 Charger is Here!
I think your misunderstanding me. A 21700 to 26800 battery adapter simply makes a 21700 battery the same size as a 26800 battery that can be used in any 26800 host flashlight regardless of what it has for threads. I have 20 amp QB26800 batteries and a lot of more powerful 21700 batteries that don't heat up when running at high levels. I want more battery options for an extended power outage.
You're looking for the equivalent of a 26100 dummy cell and a sleeve to fit 21700 in a 26800 tube? Tricky...
Other way round is a bit easier. Usually a couple of rubber bands around the cell to fit the width. Protected cells if extra length is needed. Straight out adapter, I haven’t seen one yet, or one I remember.
I want a 26800 to 21700 adapter for my K1 
Yes, I'm finding that out. Lumintop makes one for their D3 but I can't get an answer from them about purchasing separately.
Here is the Lumintop adapter
The metal piece is 10mm thick and threads into the black tube. Hopefully as 26800 batteries become more popular Lumintop or other companies will start selling them separately.
1 Thank
Cu spacer hammered into PVC pipe?
I'll have what he's having! 
The Tardis of battery adapters?
Call it whatever you want...
Just gimme a 26800 to 21700 adapter STAT! 
Thats a negatory good buddy 
OMG! OMG! OMG! AAAAAH! AAAAAAAGH!