Not fancy, but I use an army blanket tucked around the seat and seat back. Washable, low cost, looks fine. Be aware that there are low-quality ones sold as army surplus nowadays, and they will shred and fall apart. I recommend a vintage one, I found a couple on eBay.

 

NOT Coverking. The base fit like junk. The top wasn't cut for the center seat belt shoulder loop. The seat gaps had some rubbery wet suit like material. The seats and armrest wouldn't fold closed with out the rubber binding on itself. Coverking screwed around with me and played the circle talk game when I tried to return it. Ended up paying a restocking fee to return a product that didn't fit. $220 for the Coverking set.

The Subaru one is a bit pricey. It's not much more than an aftermarket. It just works.

 

I don't regret getting the Subaru one. It fits well and comes off and on easily.

 

I bought this on ebay for $23.00. Works great and has heavy duty construction unlike the vinyl ones that rip.
Pet Dog Seat Cover Mat for Car Rear Back Passenger Trunk Dirtproof Scratchproof

 

 

I use these with a harness for the benefit of the rear AC:

Amazon.com

Amazon.com

Amazon.com: Kurgo Tru-Fit Enhanced Strength Dog Harness, Crash Tested Car Safety Harness for Dogs, Includes Pet Safety Seat Belt, Steel Nesting Buckles, Front D-Ring for No Pull Training, L, Black : Everything Else

Amazon.com: Kurgo Tru-Fit Enhanced Strength Dog Harness, Crash Tested Car Safety Harness for Dogs, Includes Pet Safety Seat Belt, Steel Nesting Buckles, Front D-Ring for No Pull Training, L, Black : Everything Else

www.amazon.com