I exclusively use the Kayfun Prime for MTL, i'm also a bit stumped as to where the "hassle" to fill it comes from (and i'm through more than a gallon of RY Whore alone with it at 2ml capacity - so i filled it quite often in the last 10 months). It's closing the juice flow, open the top cap. It's two turns on the tank itself which is done in less than a second, then unscrewing the topcap, which is another two turns. Fill it, top cap back on, twist the tank twice to fully open the JFC, done.
I'll be honest, i have never heard someone complain about the way you fill it with juice (which doesn't mean it is without flaws - just not in this regard).