Well, I remember seeing a member making a camera mount that looked something like this...
He had used a steel square shaped tube, and drilled a couple holes to fit through the headrest, then had a screw going through it upward, that fit in most camera tripod holes. a 2x2 of wood would work just as good I assume. Just push the headrest as far down as you can and it should keep the bar stable from moving.
Personally, I have found this method to be effective:
Lol, I have an iphone4, wrapped in a rubber/silicone ipfrogz case. The rubber keeps it from sliding out, and it jams in perfectly!
Video of it shoved in there:
YouTube - 2006 Nissan Sentra SER Spec V Mountain Run
If you have any other type of mounts, post them up, and some info on how you did it.

He had used a steel square shaped tube, and drilled a couple holes to fit through the headrest, then had a screw going through it upward, that fit in most camera tripod holes. a 2x2 of wood would work just as good I assume. Just push the headrest as far down as you can and it should keep the bar stable from moving.
Personally, I have found this method to be effective:

Lol, I have an iphone4, wrapped in a rubber/silicone ipfrogz case. The rubber keeps it from sliding out, and it jams in perfectly!
Video of it shoved in there:
YouTube - 2006 Nissan Sentra SER Spec V Mountain Run
If you have any other type of mounts, post them up, and some info on how you did it.