For example, if a couple of hijackers steal a plane and fly it to Thailand, they will turn off the device around the same time that they turn off the transponder.
OK... then make it a device that can't be turned off, that automatically switches on if the transponder goes off or an emergency is signalled.
Place multiple ones on the plane with independent radios in hidden locations -- mostly accessible only through maintenance compartments and only when the aircraft is on the ground.
They don't need to be $100,000 boxes that tie in with the official flight instruments.
A simple GPS reading will do, with an accelerometer and a gyroscope reading -- should be possible to assemble one of these for less than $1000.
I would suggest each one have two radios, and even under normal operating conditions -- send a non-emergency position update once every 3 minutes; with aircraft identifier, latitude, longitude, altitude; in event of an emergency, 15-second updates on both emerg and non-emerg radios of all the multiple radiobeacon boxes ---- the expensive bit is the ground listening stations to record all this data!