They provide some parts, kits, and guides for replacing certain computer parts. One example of those companies is iFixIt. There are multiple third parties that manufacture Apple computer parts. If you're able to find it you could take the dimensions of it and turn it into a CAD file using a software like Blender, Fusion 360 or any other CAD application. I'm not sure if Apple patented the chassis, but if they did you might be able to find the patent. Here are some possibilities I can think of for finding a CAD file. Apple isn't known for making their computer parts user replaceable or making their parts public, because of this I would think it would be highly unlikely Apple would publish CAD files/designs of parts.
