Had to implement a new closure for my 45 litre kegs which originaly came from Grumpys.Had been using hard plastic bungs with a screw in plug where the spear would have gone before they were converted to corny style couplings.
Take a 55mm silicone bung and a "U" bolt that with judicious use of some bolt cutters became a sort of double j bolt which just hooks under the rim on either side.
Stood upto a pressure test ok and a hell of a lot easier to deal with than the hard plastic bung.
This is what the old bung was like - made cleaning a pain and needed periodic replacement. Only a few left so had to bite the bullet. 
