Like you, I also ran into metal behind the dash and dog house area where I wanted to install an outlet. The only solution is to find an area without the metal. It may not be the optimum location, but it usually ends up working out.
I did see a surface mount USB that looked like a small puck with the outlets on the side. It attached with these small screws into the rigid plastic dash but I was concern they were a bit too small to stand up to repeated use. Another option is to get one of those flip down brackets so the USB is mounted horizontal under the dash and can be flipped down when needed.
You may want to just search online for vehicle usb outlets and sooner or later one may pop up that would work.