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.
Click the 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.