WHY?
Headrest mounting system that should fit all headrest rails. Included a case for just the tablet or with the Kids case.

WHAT?
The system consists of two connectors that fit your headrest rails, two latches to secure the connectors and a case that mounts to the connectors. I recommend PETG for the case and brackets, to fend off warping in a hot car, and TPU for the latches, for its layer strength and flexibility - I do fear TPU loosening, slightly, in a hot car, due to the material softening but the design should be tolerant enough to hold (my region of dirt is cold until April-ish, will report back any issues).

The connectors do stick out of the case body, slightly, on our Highlander SUV.

HOW?
Measure the diameter of your headrest rail(s) and choose the appropriate bracket. Print each bracket and corresponding latch, twice, and then print the case style you want. Alternatively, if you have a multi-material printer or want to use them as a guide to know what a full setup looks like, you can take advantage of the full tray folders.

CAUTION: Trays are multi-material, need to modify for mono-material printers (like mine)

Print Details 

Tablet Mounts
material: PETG, I ended up using PLA-CF but that may turn out to be a mistake, in the middle of summer.
infill: 15%
support: Yes, tree
orientation: On Face, for tablet only case; upside-down for Kids case
I did successfully print at a 65 degree angle for minimal material but had to add manual support enforcers for it to print correctly.

Rail Connector
material: PETG
infill: 15%
support: Yes
orientation: Auto. (closed flat side down)

Latch
material: TPU
infill: 100%
support: No
orientation: Auto (side)