Thanks for the photos. My thought is to make up a similar bung with an O'ring that fits the washer neck (either 3d print or turn a piece of nylon rod) Screw to the top of it a disk of aluminium (or plastic) that is the correct size diameter to clip on a lid such as the one shown. As said - Unable to get a lid the correct size to clip straight on to the neck.
David


​​​​​​​