Something I noticed when I witnessed a minor accident - one car bumping into another at ~30 mph. It's more plastic sounding than you might think. Point being, something to be said for a more hollow thump in there, especially if you consider that the sound of something hitting the hood isn't quite one thing hitting a solid car, but one thing hitting an object with a complex, enclosed space inside of it (it resonates). Also, keep it kind of simple. Better to find/design the precisely needed sound than to have entirely too many layers - that can kind of cheapen the moment.
It kind of depends on if it's happening on screen or off screen. I did one film where there was an off screen car accident (lolcopout) and what helped for me was visualizing and kind of storyboarding the -precise- sequence of what happened. Because I'm kind of sick and twisted, I ended the sequence with someone being flung out a window. The result of this was I added a more dull sounding thump that could be body against hood. When I showed the director, she actually recoiled a bit and when I asked her what was wrong, she confided in me that it brought her back to an actual car accident that she was in.