Canvas Cargo Liners, OEM rubber mat, collapsible bowls for water and food, doggie waste bags and a weathertech barrier and ramp because our dog has severe arthritis.
For short trips, she gets tethered to a tie down and the back window is open. The tether (metal cable) make sure she doesn't get to frisky and try to jump out the back. It also doubles a security cable when I need to lock things up.
For longer trips we feed her a bunch of Bendryl (she's a puker) and use a collapsible crate. The she has a house when we get where we are going too.
I found the best solution was a seat cover + a proper seatbelt system for my dog. I understand why people like letting their dogs roam free in the back, the dogs like it, it's comfortable for them etc. etc. in a collision just remember your dog will become a projectile. It does not take a high speed collision to cause serious damage to anyone/animal not secured in the vehicle
years ago I had to avoid an accident at an intersection (someone turning left, I was going straight). I was only going about 60km/h (<40mph). if my dog wasn't seat belted in I am positive she would have had serious injuries and there wasn't even a collision
__________________ A mistake that makes you humble is much better than an achievement that makes you arrogant