I have had the watch for 3 years now, that use case has never once come up for me. Seems like it would make more sense to just have an extra charger in the car with one of those portable USB chargers. No need to add a permanent "charging station" for it unless you plan on doing it everytime you drive. If for some reason that is the case I would suggest the one linked below. It has adhesive on the bottom so it stays in place.
I sleep with my apple watch on and charge in the car on my way to/ from the office almost exclusively. I'm stuck sitting in traffic for an hour each way and my apple watch isn't tracking anything while I'm driving, why not?