At the moment, I would recommend getting a cheap 18650 to 26650 Battery Converter. They cost about $2 from fast tech. and is basically a plastic tube that makes the battery seat properly. VTC's are pretty easy to come by these days and do a great job for sub-ohming.
This is a great choice if you are already vested in 18650 sub-ohming gear, but defeats the purpose of going for 26650 mods.