We have constructed a special 19-21mm Rubber Bung that allows the use of a flex up to 6.8mm in diameter.
There is room on the bottom for a female cord grip to be added but it will spin when the bung is tightened up.
The bung is tightened by using a spanner on the top steel nut until a snug fit. A Nut cover can then be used to cover this nut. The bung can be fitted even when there is a flex in place.
Fits a bottle: 19-21mm
Inside diameter max: is 6.8mm