We use a Brodit one with our mini 2 but it fits to the headrest on one seat.
Little one loves it. Saves a load of headaches when travelling hours to see family.
However the holder may be used to rig something up. Saved £100’s compared to the BMW prices.
Holder – http://amzn.to/29YfGC7
The headrest clamp is available separately and comes in different sizes to accommodate different headrests. It clamps on using allen bolts and has been rock solid. i’ve used some insulating tape around the chrome headrest posts just to offer some protection and friction so I don’t need to crank the clamp up super tight.
http://amzn.to/29YfMK2
Once fitted I have never removed it. But have had to tighten the holder to the bracket bolt once or twice. But no way was it an issue.