The kayfun is rather tall, which is yet another benefit of RDAs. No tank to add height. Just a coil and wick in there. Regarding replacing a coil on the go, I would just carry a spare atomizer in a jacket pocket or glove compartment. If you use thicker gauge wire you reduce the risk of the coil "burning out".
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Yes, exactly! I haven't taken my KF out of the house yet, because it's too big to carry around comfortably in a pocket.
(Also, I haven't totally got the hang of building good coils yet -- my hands are too big, or too clumsy, or both, for this kind of work. I wouldn't want to carry around a device all day unless I had confidence that the coil is going to hold together and work well.)
Regards
Glenn