The following does cost some money, but the demands for the application I created this for required stepping outside of what was laying around for free. However, you can make one for about $70.
The requirements I had to meet were:
Support a 24in monitor and be adjustable left/right up/down
Once set in place remain very rigid and resist shifting, even under aggressive driving conditions.
TOOLS REQUIRED (I made it at TechShop)
Drill Press (or handheld drill)
Ruler (digital caliper is handy too)
(5) 1/2in Wood Screws
(2) 2in bolts with nuts
12X12 3/4in Starboard (both for looks and workability) available from McMaster Carr
NOTE one sheet is enough for creating 2 mounts. Feel free to use a cheaper material like multi-ply plywood
3 Ram Mount Products:
RAM 2.5" Round Base with the AMPs Hole Pattern & 1.5" Ball
RAM 4.75" Square Plate with VESA (4 X 75mm) (4 X 100mm) Hole Patterns & 1.5" Diameter Ball
RAM Standard Length Double Socket Arm for 1.5" Ball Bases
Links to Ram Mounts:
Disclaimer: I owe a lot of the credit for this modular solution to my buddy Josh who turned me on to the idea...
Step 1: Cut Starboard Into Slices
Starboard works in a table saw and drill press very similar to wood, and doesn't have a tenancy to melt like ABS and other plastics.
HDPE polyethylene is UV resilient offers the excellent moisture resistance of LDPE with a higher-density, firmer construction. It also resists most chemicals, such as alcohols and ethers.
Measure out 9 X 2.5in slices on the 12X12 3/4in thick sheet, making the best use of your material
Using a table saw make your cuts, and they should end up looking something like the images in this step...
Step 2: Drill Through Holes and Headrest Grooves
For reference, my bolt hole centers are spaced 8in apart.
This obviously requires pre-measuring for each car, although I found quite a few vehciles that are spaced 6 1/2in center rail - center rail.
TIP: Have a headrest with you while drilling / measuring. I took mine along with me into the Techshop so I could measure twice and cut once.
Once the correct sized through holes are drilled, bolt your two pieces together securely.
Mark the headrest rail spacing with a center punch or similar
Locate a Drill bit approx the size of your headrest rail, and increase the size from there. You can always drill out more material but you can't add it back in!
Drill the two pieces while securely bolted together, remember to appropriately secure the two pieces with a vice or similar.
Fit check the groove on your headrest rails
NOTE: I didn't take a pic of the two pieces together before I drilled them, but you get the idea. The image in this step depicts what one of the pieces would look like after drilling with through bolts sitting in place.
Step 3: Bolt the Ram Mount Ball to Plate Facing Backwards
Grab the biggest diameter 1/2in wood screws that fit in the Ram Mount 2.5in ball and attach it using at least 5 of the holes.
The thread strength of Starboard in my experience appears to be as good or better than plywood, so 5 screws is definitely overkill even for a 24in monitor.
Step 4: Bolt the Vesa Mount to Your Monitor and Connect Your Ram Mounts
Assemble the Ram Mounts together and attach the plates to the headrest in the vehicle and it should look something like the images in this step (shown without monitor for more clarity)
AND....... you're done!
ENJOY displaying whatever the heck you want!!