Steve's got the most common way it's done low budg. You can add a one inch thick (18" x 18") square of plywood to mount the head or baby, or even just run a bolt with fender washer through into the camera's tripod bolt receiver threads. Then you pull the webbing through holes in the plywood sheet.
It's about the same rig to hang the camera off of a door for two-shots or close-ups. Here, you add a little framing under the plywood sheet. It can take some adjusting of padding and rolls of duct tape (keeps the padding from flying off and sliding out of adjustment) to accomodate the shape of the door and not crumple it given the thin-ness of some modrn car's panels. Two of the rachet webs have to reach up to the top of the door or over to the door frame on the other side. It can look pretty cheezy. But, if you use some sense and LOTS of webbing rachets, padding and duct tape it can work just fine. Remember there's a camera hanging off of the car or it can be "Goodbye, camera". While that is obvious to any human being, you'll have to remind the actors over and over.
One of the most significant things is that you are using a light camera. Mounts for film cameras can get a little more involved including welded steel.
When you say "webbing" is that something that comes with ratchet straps?
I've been using A Sticky-Pod Pro (4 suction cups under base plate) with the supplied
tethers for a 2.5 lb. Mini-DV camera. I'm thinking about mounting an HVX-200 (about 5.5 lbs.)
on it. Should I buy/use rachet straps and webbing? Thanks.