I did find allot of tablet holders for on a head rest without the need of bolts or other (non printed parts).
This is fully only printed (beside a bit of glue)
I created this one for the Nvidia Shield tablet
Note however that you can perhaps design you own tablet grabbers and glue them to the hooks i made
Since they are two lose hooks the distance between the headrest pins does not matter
Pictures are made on a seat of a Toyota Auris
Print the hook "vertical" at they are vertical in the car. The print lines will then be very strong.
Print the tablet holders on there side.
When you glue the two parts together the two parts will stengthen as the printings lines are 90 degrees the opposite of each other.
On the images i used PETG filament.