I fill my 3.1 using the stock tip that comes with Liqua eliquid bottles.
I took that photo to show how the tip seals the hole but i fill it like this:
1.- Put KF with the base looking up then rotate it 45° so the fill hole is looking to the floor
2.- Instert the tip of a bottle sealing the fill hole and rotate 90° to the other side so the liquid moves to the tip of the bottle
3.- Fill until you don't see any more air(i leave a very tiny bubble)
4.- Rotate 90° to the position in step n°2 so to clear the bottle tip from any juice
5.- Remove bottle and keep vaping
