Fwiw I have used one of these
Lookout II Dog Car Seat with great success in the rear and front seats of cars as well as sideways in the boot of the Smart (not sure if the larger one would fit). I like them because it allows for safely harnessing the pup in which keeps her secure in the event of a short stop or accident, it also protects the car from her scrabbling around and she likes being elevated.
Full range of their car seats -
Snoozer Lookout Dog Car Seats