There is no "inner sheath" on an airsoft silencer. It's basically a hollow tube with foam inside. Nothing more. Use whatever you want for the outer tube as long as it's solid.
I recommend that you take a look at music (instrument) store for drum felts (the kinds used to cushion cymbals). They're cheap can be used for inner padding for the silencer tube. They already have a hole in them. Just stack a bunch of them. As for the hole, generally it's made to be slightly bigger than the size of the caliber of weapon you're replicating, and not based on the actual BB size. For example, I have a supressor for my MP5 which is normally 9mm. The hole on the supressor is 14 mm.
And remember, the silencer has to be PERFECTLY parallel to the bore of the gun, or BBs will bounce off the inside of the silencer.