3mm…so close and yet so far… According to CRX :
Tail PCB + spring are 2.2 mm. A custom insert shaped like a bucket with a nub inside could reduce it to like 0.6 mm.
Endcap is 2.54 mm thick. Removing some material should make it possible to fit 20700 without a spacer. But that’s quite a lot of work for such mod…