TAPR or CERN OHL, probably— Kit cars do already exist, though are apparently aimed at hobbyists, and usually just partial cosmetic customizations. “Metal box on wheels with motor” ain’t exactly rocket science, although quality could be challenging and that’s especially important when it comes to safety.

That said, surely the production costs of modern vehicles needed to do their basic job— Efficient-ish and safe-ish transportation from point A to point B­— Can’t possibly be worth their increasingly inflated costs? There’s probably something to be said about the marketability of a sub-$10,000 basic OHL car that you can choose to scratch build or kit-build or buy fully built.