I ordered a speedometer backing plate (part # 67320-95) and it arrived earlier this week.
It's a nice solid piece of metal. For some reason, I was expecting plastic. I was pleasantly surprised.
Here is a test fit. The backing plate sticks out a bit because there isn't a trim ring covering the entire side of the speedo. I plan to paint the side of the speedo black (it's currently ivory) so I think it'll be less noticeable when it's all done. I will probably paint the Auto Meter clamp black too.