I've got one of these in all my planes
http://www.californiasailplanes.com/...larm%20New.htm
They're very light and small,easy to fit in anything.
Our field is surrounded on 3 sides by fairly thick scrub[forest]and it's a bugger wandering around in there for hours looking for a plane,this makes life a lot easier.
The other advantage is that it gives a voltage readout of the Rx battery everytime the plane is switched on,great idea[8D]
Cheers