I have mine on a 7" mag mount and so far it hasnt fallen off... Apart from that one time when I had a run in with a very low branch
However, since Im using the car more and more for laning Im looking at drilling a hole in the roof and mounting it through there somehow. I reckon it will be a headlining out job tho.
Dont forget to let us know (With piccy's) how you get on with the install.