I used to use just a 6mm Festo with a "plug" made out of a length of 6mm fuel line that has a fuel-dot in the end of it - pull the whole 6mm line off for fueling, etc. I did once get a flameout on take-off due to an air leak between the plug and the Festo when the plu didn't seat properly, so I now use a more redundant system ... the one-way Festo has been replaced by a 6mm Festo ball-valve. That way even if the "plug" does not fit perfectly, having the ball-valve in the closed position still offers pretty good protection.

If the above is unclear, lemme know and I will post a pic.

Gordon