There are two parts to this "question."
You ask for a design to be done; but, the design is already done and, from what you say, it is functional.
That brings us to the "doing" of the design. Let's call that the "build" part of the "design-build" process.
- Wash your hands and find a well-lit, clean, clutter-free workspace.
- Remove your prototype and place it on a sheet of clean paper. Use some masking tape to hold it in place temporarily.
- Use a pin-hole to locate the centre of your intended opening. Be as accurate as you can.
- Put a pin-hole on each of the radiating index lines of your prototype. Be as accurate as you can.
- Lightly, draw a faint, rough circle with a pencil where the opening appears
- Remove the prototype from the paper "template" base.
- With a pen draw lines from the centre pin-hole outward to the pin-holes for each of the index lines. (3, 4, 5, 6, etc. to 30)
- Fold the paper in half through the centre pin hole and cut a hole with a pair of scissors to make a clean hole in the centre of your template.
- Flatten the template and glue some cut numbers in their proper position for each index mark around the aperture.
At this point, the template is finished and can be cut to the proper size and positioned on your device. If you want something even cleaner, photocopy the template onto self-adhesive or regular paper stock. Cut the aperture and then the template.