one of my friends got a form 1 on his suppressor then milled one, but his was for an HPA rig. the muzzle report was quite literally mousefarts. We tried sticking it on various things, and came to the conclusion that most AEGs don't get a significant gain out of it, as their muzzle reports were already an insignificant portion of their noise production during firing.

from having 3d printed various flash hiders and amplifiers, you will most likely find yourself making crude threads then using the threads on the gun to cut them the rest of the way unless your 3d printer has amazing resolution.

the suppressor itself is the controlled item, and even unintentional suppressors when used as suppressors (oil filters, leadthrowing airgun compensators/moderators with covers fitted, etc) fall under that definition. while they don't thought-police our actions (yet), getting caught with a mock suppressor that had been modified would likely not turn out well for you.

while it's unlikely that a local LEO cares enough what's inside the suppressor to open it, if they are in a bad mood and decide to check, you could get some serious bad juju with the government.