First, this is not my original idea. This honor goes to someone in one of the audiworld forums. However, I did improvise for a simpler design. I can't compare to the original design directly, but mine is good enough for the purpose and easier to make.
I can't find the original thread right now, I'll edit the post with it when I do.
Parts:
1x 36" 1"x1" Aluminium square tube
1x 1"x1" Black square internal plastic caps (recommended; maroon pack in picture)
1x 1/4"-20 Wing nut coarse (optional; orange pack in picture)
1x 1/4"-20 x 1" round head slotted machine screws (green pack in picture)
OR
1x 1/4"x1" steel/zinc plated wing screw (optional, white pack in picture)
1x 1/4" Med. split lock washer (recommended; not in picture)
2x 1 1/4" U-bolts [similar to SKU#629595] (adjust as necessary based on headrest post diameter)
1x 5/16" Clear vinyl tube - just need 3"-4" of it (recommended; not in picture)
1x
Sunpack Compact Ball head OR
Slik Medium All-Metal Compact Head 
Steps:
1. Mark headrest post locations on Al. tube (one post about 1.5" to 2" from one end)
2. Position the u-bolt ends equidistant from the post centerlines and mark where the holes need to be drilled
3. Drill 4 holes for the 2 u-bolts (hole size depends on u-bolt chosen)
4. Mark spot on Al. tube where you want the camera positioned (I did mount center of car but then cut distance back since the video was a bit jittery except on really smooth roads)
5. Drill 5/16" hole for the 1/4"-20x1" screw (note that these holes should be on the other sides than those for the u-bolts)
6. Cut vinyl tube into 1" sections, slit along the length of the pieces, and wrap them around the headrest posts
7. Cut Al. tube at desired length, and file the burrs/edges
8. Insert the plactic caps on the ends
9. Slip one of the lock washers on the 1/4"-20x1" screws and insert from the bottom into the Sunpack Compact ball head, tighten it to be firm
10. The part layout picture makes this step pretty self-explanatory.
However, for those who need it: position the Al. tube on headrest, insert the u-bolt ends from the other side, place the plate on Al. tube, insert a couple of lock washers on, thread on the nuts (I changed over to wing nuts because it's easier to tighten/loosen with bare hands to install/remove the headrest mount; I planned to make another mount for the SUV too and just move the compact ball head and camera over, but then I made another mount with the remaining Al. tube)
11. Re-tighten all nuts/screws and mount the camera
12. Adjust ball head angle as necessary and tighten
Here is the headrest mount on my Golf. The headrest posts are angled back, so the compact ball head is necessary. While driving, the passenger seat does shake unless there's a person sitting in it. Most of my driving/commuting is solo, so I mounted it on driver seat.
I had plenty of spare parts left over (except the compact ball head) to make another mount for the Pilot SUV. On the Pilot, the passenger seat is quite stable even without a person in it. Plus, the headrest cushions are too snug to get space for fingers to tighten a wing nut but hold the Al. tube very firmly in place (the headrests don't tilt/rise like most others). So the Pilot headrest mount is just 'rolled' into place. In the normal seating position, the camera doesn't need to be re-angled to horizontal so I mounted it directly on to the Al. tube for a even cheaper headrest mount.
Inexpensive headrest camera mount for in-car videos 
