Strange question.....

For your purposes, I think I would get a couple of large pieces of cardboard or poster board, pull the rear seat and make a stencil pattern...test fit in the car...then transfer it to plywood (3/8" would probably support your dog if you support it). Also, you might consider placing some insulation (foam or fiberglass) under the horizontal board to isolate your dog a bit from heat and vibration. I think I'd just paint the horizontal piece of plywood and toss a dog pad, old blanket or some towels in there for the dog to sit on...easy to pull out and clean...the vertical piece could be painted or carpeted. Cheers!