I have a sleeping platform we built for our Rav4. We sold the car so don't need it anymore. It's built in 2 pieces, and the front piece attaches with drop in screws so it can be quickly removed and stored on top of the rear and the rear seats used. The front also has a fold down headpiece so that you can slide the seats bike while driving if you don't want to take it apart.
It was built for our 2010 Rav4, but it should probably work in other SUVs or station wagons. The back piece is 44" wide x 33" deep. The front piece is 44" wide x 32" deep with a 6" fold down extension. Together is 44"x64" with the head piece stowed and 44"x70" with it folded down. It ends up being a little shorter than a normal bed and between a full and a twin in width. It has about a foot of storage underneath.
Located for pickup in Seattle. Asking 150$ OBO. (Dog and mattress not included)