Thanks. It's a $12 eBay LED taillight, which I chose because it was cheap, small, shaped to fit with the bike's styling, and lightweight. It's brighter and less weight than the Lucas-type knockoffs you get from cafe racer suppliers.
I've used these on other bikes, but the manufacturing quality is iffy (probably soldered together on a Chinese sidewalk after dinner). You sometimes have to take them apart and re-solder a wire or two, and the mounting bolts are fragile. If you get a good one, it will work fine. I also have an FZ6 taillight that I've narrowed to fit under there.
I mounted the light on a piece of sheet aluminum that I had cut and bent to fit onto the frame tubes behind the crossmember. I can't remember the gauge, but it's thick enough that I had to bend it in a vise. Any thinner, and the light and license plate would wobble. This acts as a mudguard. It keeps the bottom of the seat dry and also holds the license plate. The plate is painted black and bolted to the frame tubes:
Here's a before and after view:
Good luck with your project. You really can't go wrong with a $1,500, seven-year-old bike.