MeshMixer is the program I use for a lot of mesh editing. It has a nice set of features, but feature development was ended in 2021, so there may be more modern options available.
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If you just want a straight cut I'd think your slicer would be able to do that. Prusaslicer can, at least.
After doing it in TinkerCad, it does look like the slicer could probably have done it too. Thanks!
Tinkercad. Paste it in twice. Edit away half of each, stick together.
That is s fun little tool. Worked great, thanks!
Try blender. YouTube tutorial.
What software are you using? Do you have any experience using any CAD-program?