How big? I have had "car seats" for my dog. If I had the seat in the front, I latched her to the seatbelt. I didn't always do this in the back because I often drive very long distances and liked her to be able to lay down and sleep in any position she wanted. I know my butt gets tired after a while and figured hers would as well. I've also put regular small dog beds in the seat when that made sense with what I was carrying. My dog wasn't the scratching type but I still protected the seats all I could. I've used items like this:
https://www.amazon.com/Snoozer-Look...8&qid=1536542303&sr=1-5&keywords=dog+car+seat
Honestly, if you're at all crafty, you might just find some vinyl (it comes fabric backed) at a fabric store, glue or sew some loops that will attach it to the headrest correctly and make it custom for your car. I've done that with my dog beds - made them fit exactly the space I wanted with foam.
One more thing. They make clips for dog harnesses to attach to seatbelts. You could probably pretty easily adapt that to attach to the car seat anchors in the cargo area. There are also 4 anchor points on the sides in the cargo area that could be used as tethers. Sometimes, the right solution isn't what someone has on Amazon. (Can't believe I actually said that - LOL - we're very best friends)!