The clone subtank mini top fill tanks work well, but the fill holes are only around 2 mm wide. I like the short variety my self. They are 100% compatible with Authentic Kanger Tech Subtank mini bases and O-rings.
subtank mini tanksHere is the what I like with clear or black O-rings and a POM drip tip!

If you want the authentic
Authentic KangerTech Toptank Mini Sub Ohm Tank Clearomizer , but I have not tried it, and am extremely happy with the Fasttech versions.
Edited on 3/30/2016 at 2:35 PM. Reason: