A few extra large garbage bags have always taken care of dirty bikes in the car. If you do it a lot then a soft bike bag would be useful or get a canvas bag made up.
I prefer putting the bike in the car when on my own, except my car is now too small and besides the hatch is full of amps.
The X5 is my wife's car. She is short too so would never be able to reach a bike on the roof.
This is mostly for a quick ride to the bike track on weekends with the kids, rather than heading bush.
I like "Crackinfails" a fav bike, I used to race them, Vets, in XC and DH mid 90s. In those days it was the same bike for both events.