The Constitution doesn't stop anyone. A fair Judiciary shouldn't allow it and I think most of the States would not approve either.
The 'loophole' is an intrastate transaction between private parties. The Feds have no role. They can and do pressure the individual States like they do with seat belt laws.
An FFL can do interstate transactions and that is why they are subject to federal regulations.
While, in theory, I consider you to be correct, in practice, it doesn't matter.
What the Constitution allows and what the Congress passes don't necessarily jibe.
Refer to the '94 ban...it was any weapon that has ever traveled in interstate commerce or contains any component part that has ever traveled in interstate commerce.
Unless all parts of a firearm were originally manufactured in your state (assuming they don't intend to go as far back as the raw materials entrance into the supply stream...iron ore, raw plastics, etc.), the feds will consider your firearm to be covered by the ban, Constitutional or not.
Your comment about a fair judiciary assumes 1) such a thing exists and 2) people should be willing to be test cases and risk their life and liberty.