A standard plate bracket works fine. Most VW places sell this one. If you're concerned about the curvature of the rear of the car, you can use nylon spacers and longer bolts to space it out a little. Then twist the tabs of the bracket so it sits more vertical.
That is my car with a vintage NY plate on it. The plate is slightly larger than the current ones by about a 1/2" in both directions.
Pay attention to clamshell open exhaust clearance as well.