Honestly, what you need to do is get a thin rubber tube - something a metre in length and a bit thinner than a straw.
Thread that through the eye of your knob as far as it will go and stop when you feel that rubber nudging your balls. Then blow until your sack starts filling up. Once done, place a few drops of super glue in the rubber tube and blow qucikly so it seals the air off and stops it from escaping.
It works!