Tyre pressure sensors - need to be on the valve?

[s2sol]

I've got a 407 with tyre pressure sensors, and the valves snap as soon as you look at them. I've had all the valves replaced with cheap ones, and have the sensors rattling around in the glove box. Someone has suggested fixing them inside the rim somehow. Other than balancing issues, is this viable? If so, what's a good product to use to fix them?