I've had one flameout on take-off due to an improper seal in the filler line, so I elected from then on to use a double-redundancy method.

I have a festo ball-cock on the fill-line, and a "stub" - a small (about 2 or 3") length of the 6mm tube plugs into that with a good quality fuel-dot in it.

To fuel the model, I remove the small length of line, and open the ballcock. When it's full, I close the ballcock first to prevent air getting in when I pull the filler line out, then I put the stub in place.

Sounds complicated, but it isn't really, and it has worked extremely well for me. If the stub isn't properly seated, the ballcock prevents leaks ; if the ballcock isn't properly closed, the stub prevents leaks.

Gordon