Flatten 3D Waveguide to 2D

I created a waveguide in ATH 4, which automatically generated a 3D STL model. I'd like to extract the curve of the waveguide so I can make a tool (cut a piece of plywood) to shape a waveguide from plaster. Does anyone know how to flatten a 3D STL image so I can print out the curve to trace onto the wood?
 

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    Screenshot 2020-10-19 094032.png
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