When my son came home with this tablet car mount he made in school, we ran with it as a nice father and son project...

We ended up making two mounts together...

Step 1: What Do We Need?

Material:
Wood plank (25*20 cm or 10*8 inch, 18mm or 11/16th thick)
Aluminium profile
5 small screws
Webbing band (15mm or approx 1/2")
Buckle
2x 50cm/20" Bungee cord (3mm)
4x 3mm clamps

Tools:
Hacksaw for metal
Power drill
Dremel
Screwdriver
Glue
Router bit for Dremel
Grinding bit for Dremel
Yardstick
Pencil/market

Step 2: Preparing

Measure the approximate distance between the stands for the headrest in the vehicle (ours was 10cm/4") and measure half of that distance to both the left and the right of the center.

Then take a little more than half the width of the webbing band, and measure that above and below the center of the two lines you just drew...

Also shorten the aluminum strip to the width of the mount (25cm/10" in our case) and round off the sharp corners with a Dremel grinding bit....

Step 3: Make Two Slits...

Use the Dremel with a router bit to make two slits in the multiplex board for the webbing band to pass through

Step 4: Mark Drill and Glue

Mark off equal spaces for screws (remember to halve the distance for the two outsides) and drill holes in the aluminium strip, and then use those to mark off the points to drill pilot holes in the multiplex board, apply glue and join the two... Don't forget the screws! ;)

Step 5: Strapping!...

Join the webbing band to the fixed side of the buckle, and sew it in place, then weave it through the slits.
Use a lighter or a hot knife or soldering iron to melt the ends of the webbing to prevent fraying!

Step 6: Bungee-jump to the End...

Join the ends of the two pieces of bungee cord using the clamps, ensuring there is sufficient tension to hold the tablet in place once they've been strung over the mount...

Step 7: Mount Mount

Mount the mount in the car by wrapping the strap around the headrest stands and pulling the webbing strap tight (make sure you've got the sliding part of the buckle the right way round or the webbing won't lock tight!)...

Now place your tablet and enjoy!