[makeitdouble]

There's a flurry of battery holders that accept 21700 cells, but the power delivery is just too weak IMHO.

From memory, the max output was about 20~30W for the bigger models [0]. For 2 cell types it gives 10W, so barely good enough to slowly charge a smartphone.

https://www.amazon.co.jp/PLCPDM/dp/B0DFPKXQZ1

[datadrivenangel]

You can get these for 18650s as well.

Most of them use rails though, so different cell chemistries are a bit of a risk over time.