You could generate it from the Godot editor, using the mesh sub menu. It will automatically create a collision mesh based on your mesh geometry.
It is intended mostly to create debug collision meshes, but it might cover your case.
MeshInstance object and then the following sub menu:
Which results in:
You can se the faint blue line of the collision shape cutting across the prism.
Be aware that this will also generate a static body, so you might want to check what to keep and what to delete.