Marc,
Nice to see you looking at some new items, as well as taking input from users.
Transporting my models most of the time means resting the fuse right on its belly, so for me a flush or better yet slightly recessed fitting with or without the plug/cap is critical. I don't want any pressure on the vent in transport.
I've been using B&B Fuselage fittings (
http://www.bennettbuilt.com/shop/Tank_Accessories.html) and install it so the threaded plug is just shy of the surface. With the plug removed tygon tubing easily slides onto the nipple for connection to the taxi tank.
Here is how I've installed it in the past:
http://www.rcjetaddiction.com/forum/...ent-drain-plug
Certainly this is a home-made mood and there are multiple ways to achieve the same.
Another point is that for scale models the less obvious the vent connection the better.