Is there a case because, to be frank, I do not think that such a ruling would occur. Considering what the court decided in Wickard v. Filburn, the idea there is any limitation on the commerce clause is basically gone. In that case, the 'product' was not even being sold at all or even traded. Far beyond the idea that manufacturing is not directly tied to commerce, the creation and consumption of an item entirely by a singular person being classified as falling under the commerce clause is truly insane. Under that precedent, manufacturing certainly would be considered under the commerce clause. AFAIK, this case has not been overturned.
To be clear, I think that case is one of the worst precedents ever set by the SCOTUS.
As to the core objection, I am not sure what spurred the original seatbelt regulations but I can see a reasonable argument that normalizing regulations throughout the states so that CA does not have a stricter standard than WA is ensuring the free flow of goods across state lines. Why would that be beyond congressional power to enact under the commerce clause?