There are cases you can buy for your portable DVD player that hang on the back of car seats ($20), but they typically go on the back of one seat and not in the center, so both kids can't see the screen equally. Also, these holders are their own carrying case, so they don't fit into other baggage easily, which means that it's one more thing to carry. This is not so ideal when you are doing a combination of flying and driving on a trip with the kids. Finally, I am cheap, and have a hard time buying things that I could make better!
I made one from an old canvas bag (our house is bursting at the seams with extra canvas bags we get for free). The holder hangs the DVD player when it is open flat in between two seats, and the straps just fit around the base of the car headrests. If you are lucky, your canvas bag will have nylon straps, which you can reuse for the holder.
The DVD player we have is a Sony DVP-FX930 9" model that flips open so that it is flat, but you can do this for any portable DVD player. Its dimensions (with the battery) are 8 3/4" wide, 6 1/2" deep, and 1 1/2" high.
What you will need:
-old canvas bag that is bigger than the DVD player (i.e., the open DVD player can fit into the whole bag)
-nylon strap, about 3' (you can buy these from a store such as the local hardware store or arts and craft store, but sometimes you can also steal this off of canvas/plastic bags that places give away)
-elastic band, about 12" (a little longer than the width of your DVD player)
-2 sets of plastic side release buckles, 1" wide (should match the size of the nylon straps). Can get these from a local hardware or craft store
-sewing machine (though you could do this by hand if you need to)
-pencil or chalk for marking cloth
You'll need to know how to use a sewing machine. The total cost was ~$5, where I bought only the buckles and elastic strap and scavenged the rest of the materials from stuff around the house. If you are lucky and have an old bag with buckles, you may be able to scavenge those too!
This an entry for the "Are We There Yet?" challenge.