Try the Kayfun V4. Air flow is very free for an RTA and it holds up to 3.8ml. The build deck has 4 screws that let you build on one side or the other, or you can build diagonally. It's meant to be a single coil atty, but lots of people build it dual coil and with twisted dual Kanthal.
Leaking has been a problem for some and not others. Those who replaced the lower o-rings with the next size up say that they have no more leaks. I haven't done that with mine yet, but I can tell that the stack o-rings are too small. You can barely feel the o-rings touch when you screw parts together. I like to be able to tell when the rubber hits steel and have a fraction more rotation to compress.
There's a noticeable difference in flavor and vapor, and much easier draw and air flow between the Kayfun V3 and the Kayfun V4.