This was a fun project on my first motorcycle project, a 1977 Honda CB750K. I replaced the stock rear fender with this sheet metal part I designed to hold my license plate and a new rear taillight. I think it worked out well and thought I'd share the part drawing for others.
Here's the drawing for the sheet metal part. I had mine made out of 14ga SS. Take it to your local metal shop with a press brake and have it made for your bike and let me know how it works.