USB Power On The Go!
You’ll need to know how to solder for this project, and you’ll need one of the aforementioned bulky USB-to-car charger adapters, but it actually shouldn’t be that time consuming of a project. Plus, it’ll actually create a second power outlet, so you can charge more than one device at a time. The hardest part of the whole process is probably going to be fashioning the cover for the USB port, and that’s really if you want it to look pretty. Hit the link for the full instructions.