A few years ago the ATF reported that they only only prosecute 20 or so NFA crimes a year. And almost every one it is a paperwork crime. They wouldn't elaborate on that, but every so often there are reports of manufactures or dealers getting raided and full auto guns are either missing or there are several present that are not on their books. A few years ago I became aware of solvent traps at the same time the Feds shut down a company selling them. And that campany were being stupid about it. It was clear in their ads it was for a DIY can, just were not calling it a can to keep you off the grid. Wink Wink. "Works great with your 556 and 308 engines!" And the dumb thing is others have not learned. Saw another company pulling the same stunt FaceBook last week. I work with a retired ATF agent. Cool guy and some interesting stories. The thing to remember is they are small when compared to federal LEOs. And most of the time they are looking at manufactures to FFLs to keep guns from getting into bad guys hands. So if the ATF is interested in you as an individual, you did something that got their attention.
Flying, I would call the ATF and explain what you want to do and what their stance is. Or check the website on their FAQ and the NFA handbook. I know they got snotty about keeping spare wipes for cans that used them. But with the explosion of modular cans hitting the market, it does seem contradictory that one set of spare parts is bad and another is good. Especially since one is only good for a magazine or two before needing replacement. But getting them to give an opinion on how to build a can yourself legally is the best advice I can give.