Just customized and printed this today, seems to work perfectly. I might put a small bit of foam in the bottom to remove the tiny bit of 'rattle' they have due to a little extra space (about 0.5mm) vertical.
Dual 21700 Battery Box by zombienerd
Figured it may help out some other folks!
Dual 21700 Battery Box by zombienerd
Figured it may help out some other folks!