Also shot from a motorcycle, from the tailgate of a station wagon, done that a lot, from the back of a pickup but they tend to feel every bump in the road, I've leaned over the back of a flatbed with the camera mounted as low to the ground as possible and had an extension eyepiece set so I could look down while shooting (this was a motion camera) we shot from other rigs all the time. Also suction cups and using several of them and then using electrical emt tubing flattened on the ends with a hammer and then drilled to bolt into a system of triangles to get the camera out where you want it. I've also built out of wood a beam that went across and just above the hood with padded ends and strapped to the wheel wells with those smaller 2" trucker straps and it worked well.
I also watched some riggers one day use the emt idea and went under the car and in the wheel wells, attaching where ever there was already bolt, this was pretty amazing and they were incredibly fast, and could get the camera anywhere you want it and motion cameras are pretty heavy and bulky so there has to be lots of reinforcement but it is done all the time.