Gads, I hate to be a party pooper but my complete and utter paranoia of DHA and BATF is kicking in -
I suspect fuel filter moderator will do some good on an air rifle, but do be careful with the legalities. My understanding is that if it can work on a Powder Burner, it's probably considered a firearm (completely stupid, but politicians wrote the code). Even if you never mount it on a firearm, it's a suppressor, therefore, a firearm in its own right.
The preppers advertise them for firearms use, so if someone got caught with it and had powder burners in the house, the DHS or Alphabet org might consider it usable on a firearm, so could land the owner in a heap 'o trouble.
If you are buying off Ali Express, there are a few that are illegal (yes, I even checked with a lawyer) without FFL import papers, even though they are sold as oil filters. Even if there's no hole or way to mount it, if it looks like a suppressor, smells like a suppressor, and quiets like a suppressor, it is a suppressor. Just be careful and stay within the law.
Have a look at this one:
https://www.youtube.com/watch?v=270sLN5WL2I
If you believe the video, DHS follows Ali Express purchases that could be used as "Silencers". This guy apparently bought filters / adapter from overseas, and DHS showed up to collect them.
I've seen stuff on Ali Express and DH gate that turns a certain 9mm pistol into a full auto, advertised as "Airsoft Parts", or "Oil Filters" that are suppressors, just labeled incorrectly to throw the Feds off, and the Feds know the game extremely well. All sorts of stuff available that is of questionable legality.
Yes, I'm paranoid, but it's not worth the problems a group like DHS can cause.