CGAMT:

I used to use those things, but had too many of the internal valves partially stick in the open position, which results in a fuel/air leak and a deadstick landing. Everyone I know in the hobby has had the same experience, and I really would not recommend it. I've settled on the "two clunk" system. There's a third hole that can be made in the tank plug, and I open that up and attach another clunk to it. The line attached to that is used for fueling/defueling. I run the line out into the cowl, and make it long enough so I can get it outside through the engine cooling hole. Then I just put a plug in it after fueling or defueling. Much easier to use and more reliable than anything I've tried.

It is also possible to not use a second clunk, and "T" into the engine feed line between the needle valve and the tank. With this setup, though, you must be able to "pinch" the needle valve line closed when fueling/defueling. (The part of the tube between where you "Teed" and the needle valve.) I have this setup on a couple of my planes and it works well.

These are the parts I used, from towerhobbies.com

LXFU81 "T"'s
LXD747 Plugs

My Camel and the Nexstar trainer use the second clunk system, and the Mustang fill line is just T'd into the engine feed tube. You can see the little "window" I cut in the cowl so I can reach through with a needle nose pliers and pinch off the carb line when filling/emptying.