Fuel filters, oil filters, 2 liter coke bottles, etc., are not NFA regulated items, so they can be bought and sold without all the Federally required regulations and taxes.
There are a lot of things you can add to the barrel to reduce the sound. HOWEVER
You need to be aware that, under US law, it is the intent to silence a firearm (without prior govt approval) that is the crime.
The silencer doesn't need to work, for you to be found guilty.
the people selling these things are preying on the gullible and the underinformed.
Buy all the fuel filters you want, buy them from NAPA auto parts or from some slick marketer on the Internet, its legal. Put one on a gun, (without PRIOR ATF approval) and you're breaking the law.
Also, be aware of the possibility of "constructive possession" charges. This is a legal concept where, if you have all the parts needed, but don't actually do it, they can still find you guilty.