A reference square to help with accurate CAD tracing. Just include this 20mm x 20mm square in your photos or scans of objects you want to trace.

Then use your CAD's calibrate or scale feature to match the square to 20mm, ensuring your image trace is accurate.

The model is made such that it can stand up on its own, but feel free to edit it to match your use cases.
One advantage of this approach is that it automatically accounts for your 3D printer tolerances.
Place it close to the object you want to trace to reduce lens distortion effects.