The 3D model for the part you are looking at exists. I used this to make a modified back cover (“outer mold line”) with screw holes in it, so I could attach the battery extension with screws through its custom battery extension holes and also into the hole you’re looking at below.
I think it is here (Librem5-Chassis · master · Librem5 / hw / 3D_designs · GitLab) that they have a file called Librem-5.step that has a giant complex geometry file with each of the component parts that can be split out.
(If interested in the geometry I created that is described above, see included screenshots of this contraption)