I am sure it varies per manufacturer. With this swaro scope it took a 2 inch pvc cap. There was a little play room so to snug it up I usd some foam tape on the inside of the cap.
Once you get it on you will have to use your zoom on the phone so you do not get that tunnel look.
TheHunt- I think there already is one since I stole the idea from something I saw in a cabelas catalog