As another poster mentioned the eGo one has CLR heads which can be easily rebuilt, which basically turns the tank into a regular RTA. Although the eGo one is 19mm diameter but 1mm off shouldn't be too noticeable on the Istick when you consider it will be split on either side. There is also a smaller diameter eGo one tank that is 16mm I believe if you prefer something even smaller.
There is also the subtank nano, although the RBA deck for the larger subtanks doesn't fit the nano so you would be stuck with factory coils.