Ideally, just find an old car and fall against or roll across the bonnet (hood) whilst it's stationary, should give quite a good sound as a basis! If that's out of the question though, see if you can find a metal tray, like a tea tray or something. Bash that against your knee, then pitch down the sound a little bit.
Skarik's suggestion is a great starting point! While you say the car is moving fairly slow, depending on the director and how you approach sound design, think about the emotional impact of the event, and whether this is a turning point in the story, and let that influence your approach. You might want to sweeten the impact with more low-end "meat," go really "hyper-real" or descriptive in lots of little layers (hood, clothing, zippers against metal, broken bones, etc.), or even be minimal and super-understated/barely-audible based on if there can be a nice transition from car noise to post-event silence, all being dependent on the picture edit, of course. Where the camera PoV is at the time of impact should also determine which approach will "sell" for the listener while carrying the story forward.
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.
I recently worked on a film with a similar moment - A fight scene which ended in the lead being thrown onto a car hood/windshield. A second what Skarik mentioned - recording actual bodyfalls onto a real hood provide a great launch pad from which to add a variety of layers. For me, I ended up going with a combination of some low frequency elements (various thuds/impacts/etc), along with some recordings I captured of smashing different objects against a metal trashcan. Its really the balance of the individual sweeteners that helped to sell the sound along with the picture.
Isn't a body or large object hit to a some kind of a surface quite obvious? Authentic thud would likely come from some kind of a light metal surface, but something usable could very well come from something else as well.
However, there are other sounds that contribute and would likely be timed very closely to the actual hit. The car brakes, reactions of bystanders, if there are any, and the reaction of the girl. Unless there's a reason for these sounds not to be there, or they aren't there to start with. So the thud might just be one of the sounds in the combination that gives the right impression.