I bought one of these for my old Galaxy Note 4. My current phone is the same: Galaxy 8+. It fits in this as well.
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
I upgraded my phone charger to support USB-c when I got the 8+.
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
I don't yet own a tester for USB-c, but the USB A plug I tested and it drives 5.15V which is within specification and will not burn up your battery.
I also bought a USB-c to USB-c cable.
I had an old scrap of velcro I put around the windshield mount. I gathered up the slack in the cable with it and left the end with a few inches free to make it easy to plug in.
The adapter lives in the passenger footwell plug.