Reflector template
thingiverse
Print it out(to scale), cut and glue it to a piece of metal, I used 0.75mm steel plate. From there you can locate holes and the exact diameter to finish the part.
Download Model from thingiverse
With this file you will be able to print Reflector template with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Reflector template.