How do you ban body armor?

Apparently NY is about to.  Other states have but can't remember which off the top of my head and can't be bothered to search.

That raises a question, however.  Body armor consists of a carrier vest and plates.  

The carrier vest is a simple construction made of heavy material, outfitted with pockets.  I could probably make one in three or four hours if I started with a rough plan and some material.  I have several old Singer machines that I'm sure would sew that heavy material.  So step 1, complete. 

The plates are pieces of steel that have been curved for convenience and comfort and coated for extra safety against ricochet and spalling (bits of metal flying off when hit).  I could make flat plates with readily available tools (I probably have most of the tools now but if I wanted I'd go buy slightly heavier duty tools at Harbor Freight).  I'm guessing I could buy an acetelene torch set to heat and curve the plates although this probably isn't the most efficient method, but I think it would work.  Then I could get them powder coated in town for about $5 per plate.  To inhibit rust.  Not sure what coating would help with spalling, etc or if this coating would be easily applied at home, but that's not really the point.

I've just finished making my body armor.  How does the state define it to be illegal?  Does it require a vest and plates to be illegal?

So what if I put a chunk of steel in my backpack.  Protects my back from stray or targeted rounds and in a more serious situation I could swing it around to my front.  I have body armor.  Is it illegal?

I buy a couple thick paperback books.  AP Calculus study guides come to mind.  In fact, I already have them.  I sew a pocket into my backpack and insert said books.  Not as effective as a steel plate but pretty effective against a low velocity or spent round.  I have body armor.  Is it illegal?

I envision a new industry where you can buy vests with pockets (perfectly legal) and curved steel plates (perfectly legal), just not at the same time.  And it will be more of a pain in the neck than it currently is because you will have to figure out what you have to buy rather than just clicking the button that says "Buy body armor".

Oh, and this won't stop a single mass shooting because turns out no one has ever killed someone by attacking them with body armor.  That takes a weapon of some sort.