Have you considered a small device like the Wismec mini 80 watt it is internal battery and a rather small profile or say like the Council of Vapor 40 watt mini mod? Truthfully most of the internal battery tubes you will find on the market are going to be 25mm in girth and the height of a normal 18650 mechanical or they are going to be an all in one system with a non-removeable
tank system unless you go with the I-Just II kit which is a mechanical battery with low ohm and low voltage protection but offers no power adjustment.