The Plastic Carbonation Cap is Compatible with the Fermzilla and has been designed to suit bottles such as plastic coke bottles to be used for a sly roadie.
The thread specification on this product is the PCO 1881 (AKA Coke bottle thread)
It's handy to have a few of these laying around as they can be used for Beer line cleaning. Fill a plastic bottle with ChemiPro Oxior some EnzyBrew mixed with water, screw the Carbonation Cap on and connect your Liquid (Black) Disconnect and line, flip the bottle upside down, turn on the tap and give the bottle a squeeze, you can leave the cleaner in the line for a few minutes and then do the same thing with a bottle of water to flush out all the cleaner.
To take a sly roadie you would pour your Beer into a plastic bottle, squeeze the bottle so the liquid is at the max level and then screw the Carbonation Cap on and attach your disconnect and regulator to inject some CO2 in there, leave the Carbonation Cap on and take your roadie out with you!