It isn't to hard to to, just think about it and you can figure it out.
There was a group in my area that had this happen to them to be honest. A kid was offering to make them for his friends, and a neighbor or someone saw him out in his yard "testing" the products. His parents are now faced with a $10,000 fine, they were lucky that they didn't have to do jail time.
And yes I know that you were talking about an airsoft suppressor. But in the eyes of the BATF if they are not solid they are illegal. Even if they have foam in them, as long as they will drop the noise by 1db then it is classified as a Class 3 weapon.
Now I know just about all of us have them on our rifles, and have made them. But this really isn't the place to be making a "how to". The forum doesn't need that kind of attention in my eyes.
There are plenty of other places to go and see how it is done. That and there was already one put on here by the founder of the site, so just go off of that and it will give you an idea on how to do it. No need to have everyone putting up there "design" on the forum.
Now I know that is just going to go over a lot of peoples heads, but hey if you get caught don't whine about it. You knew before you made the product what the consequences could be.