Not custom... you just need euro tail lights. Rear fogs are standard on european market cars.
Otherwise, any bright rear-facing light will do. I have seen people mount them inside the rear glass, under the bumper, inside a rear reflector... and so on. Once you get the light you want, just wire in a toggle switch and go (or splice it into the front fog lamp circuit)
An easy way to keep an OEM look might be to replace one of the reverse light bulbs with a high power red LED optimized for a narrow focused beam, and then just wire it up as desired.