First, there will ALWAYS be some bounce, even with the most expensive glass move-and-shoot rigs. The good news is that some bounce will not affect the sharpness of the shot. Don't ask me to explain why because I can't, but I know it to be true. So, just because you're getting a little bounce during the shot doesn't mean that it won't turn out sharp.
Second, controlling this bounce is part of becoming a seasoned rigger, as there are many ways to do it.
- use a shorter boom and a wider lens
- use a shorter boom, normal fl lens, and only use the hood/trunk
- make/buy a stabilization cable
- seek out super smooth roads
- make/buy a cable pull
- hold/stabilize the camera with your hand while the shot is being taken
- move the car super slowly with a longer exposure
- make sure the engine is off
- remove any leaves/stones/sticks from the road
- spread the cups out as far as possible
To get a great result you are going to need to do some work, simple as that.