The vast majority of the structure can be easily 3D printed or injection molded. To match the exterior though I disagree that CNC milling would be the best solution. You only need the exterior surfaces to the chassis to match, so sheet metal with a single bend is all that’s realistically required. Like another poster above pointed out, all you need at that point is to grit blast, then optionally anodize for durability. For a small business there are companies that do sell custom bent/finished sheet metal for small batch manufacture; as with anything this will have to be balanced with demand.
A well equipped DIYer with printer and laser cutter could realistically handle printing the inner frame in addition to cutting and bending their own exterior sheet metal. Home anodizing for batches is trickier but in my opinion optional for the task of matching the exterior surface finish of the laptop. You’re going to get scratches no matter what, and the little cards are easily replaceable and repairable for a reason.