@Dirty_Harry, @ngocle, it's actually in the original two pre-made coils that come with the Protank. I'm using it with the Magneto Telescopic Mod
. It works fine if I don't screw the Protank all the way down, but the it's loose and tends to rotate. As soon as I tighten it up, I get no airflow and end up sucking in liquid. So I've had to trim the rubber(silicon) when it's in a compressed state. I do this by pushing the centre pin against a solid piece of metal until the the rubber expands out, then rotate it while holding a blade against the metal tube to cut the rubber.
The rubber piece I am referring to is this: