They say pictures tell a story so please see below. Was in the process of cleaning out and preparing the inside of the tanks and needed to blank all openings - after much head scratching discovered that a tow ball cover works great for the filler openings.
As a separate point, modification to the sender fitting was made due to the original "lock ring" being corroded, (and I also personally didn't like the arrangement). Photo shows this modified lock ring holding in a temp blanking plate, (which I finished sizing on a lathe hence the blanked centre)
Maybe useful info for others - if not already known.
