The Kayfun 3.1 ES has a fill hole on the side, however it's useless and a lot harder to get something to fit in there. Overall its a terrific little device. I got mine for $18 + tax shipped. In my mind it's well worth it. I'm going to compare to a Squape, but I can say I like it more than any Genny I've used. Hands down.
To fill with minimal leaking do the following.
1) Place your finger over the adjustable air flow hole (left hole) and the primary air hole. Make sure both are fully covered, don't uncover until directed.
2) Take a test pull of air, you should get nothing.
3) Unscrew your top cap.
4) Fill to just below your threads. Go slow with this as there is still pressure in the tank area and air pockets can form. Let them settle and continue to fill.
5) Slowly place the top cap back on, screw it down until just before o-ring lines up with the tank section.
6) Turn the Kayfun upside down.
7) You will see an airbubble shift from top to bottom. Once it completely changes direction screw the top cap down.
8) Release your fingers from the air hole (over a paper towel).
9) A small bit of juice might creep out if you had some in there previously. This is ok.
10) Turn device around.
11) Take paper towel and place over your mouthpiece area and blow into the side air hole to clear up remaining juice in the air chamble.
12) Take a small drag from the side of the airhole.
13) Enjoy
Or just watch a quick video I did on this exact process to make it a bit simpler.