I would also utilize the head to battery tube concept like the Amutorch VG10 and offer both size battery tubes 21700 and 26650. look at the last picture .here Keep the head size the same as the BLF GT Mini, I am running a poll on a similar concept with the BLF GT mini, maybe that will also give you some ideas. Who would buy a BLF GT mini with a 21700 or 26650 battery capability, along with SST-40 same size head.