Ok, on my hot rod L6, I took the plunger down from 11.0 mm to 8.9 mm or so. The 26700 now fits, though a little tight - maybe another 0.5 mm would be better, but I'll use it this way.
-> 5.7A now at 4.15v on the KK 26700 cell, on a 4C tint, doing lumens of 1,445 @start, 1,384 @30 secs. Like'n it 
, specially with 5300+ mAh in a light the size of a C8.
Could do better though on a UCL - gotta check if I got a spare one...
Details on the L6 stock and mod here: https://budgetlightforum.com/t/-/26046