18650 battery holder reliability
There exists aluminum battery holders where the only thermal weak spot is the insulating washer between the contact and the body. The washer is a glass-filled resin which should be better than the material the regular carrier is made out of, and also gets the benefit of the aluminum body heatsinking it. A bit pricey though.
Get versions where you add your own lead wires.
http://keyelco.com/userAssets/file/M65p20.pdf