Manifold issues with the mesh

[rubot42]

I tried editing this in FreeCAD and noticed that the mesh has some manifold errors. Here is a screenshot from Meshmixer pointing out where the errors are.

To reproduce:

• build the models with cmake (same commands as readme)
• convert to a .stl with openscad (same commands as readme)
• import stl into Meshmixer (https://www.meshmixer.com/)