I used to use those fillers on all my cowled planes years ago but over time they can and do leak or suck air, it happens most the time on planes that don't get used very often. The last one I had in a plane the little filler got stuck in the body of the unit and couldn't be removed. That was the day I went to the three line system.
If you don't mind seeing a small bit of fuel line on the outside of your plane then just use a two line with the carb line running on the outside of the fuse, cut it and use a piece of brass tub to attach the two halves together, shazam!! Your own custom filler, fool proof and free.
Fuel dots are just cleaner looking.