I push my Plus 8 out of the garage backwards by pushing on the top edge of the cockpit where the ash frame is just forward of the rear wheel arch that way you can steer with the other hand if you need to.
Moving it forwards I push on the spare wheel, or if at the side of the car on the dashboard crash pad, handy for steering again. In 14 years I have not noticed any adverse effects doing this, however it does remind me every time how light these cars are. I would struggle pushing either of the tin tops with one hand.
