When I faced with this problem...I dug up some scrap fabric, 2 old pillow sheets, some batting and dusted off my sewing machine. Continue to read if you are curious what I ended up making. :) (This post is still incomplete..sorry.)
I purchased some craft batting to add to the "stiffness" of the fabric. This is completely an optional step. But if you would like to do it. Measure out the batting (or trace like I did). Cut and pin to the "wrong side" of the fabric. Make sure to trip off the extra batting. Then sew.
On the back piece of the seat protecter I attached a pillow case. I figured, "while I'm at it, I might as well make some extra storage for the car." After all a Sx4 is a tiny tiny car!
Pin back the sheet so it does not get stitched into the side hems. Then procede to stitch up the 2 sides. Leave about 1-2 inches unstitched when you approach the top of the project. This will be your top "hem". I stitched the pillow sheet to the top before hemming around the top of the project. Then I measured one inch in and pinned. Stitching all the way around the top.
Stitch around your lines. After your lines are stitched around cut a slit in down the center of your "rectangle" you just stitched. This is how you are going to hang the seat protecter to your head rest.
The completed project coming soon!