Hmmm. The front of the magwell… I like that idea.
Since, according to the ATF’s NFA Handbook, the information must
“ …be conspicuously placed on the frame, receiver, or barrel of the firearm by engraving, casting, stamping (impressing), that is, they must be placed in such a manner that they are wholly unobstructed from plain view.”
That seems like a great option.