Hello,
A Bell Target take a hell of a beating at 6 yards with anything other than a specific 6ft lb target rifle.
Your 4" disk will need to be made out of something more substantial than a baking tray if you are to get much life from it.
The simplest one I have seen is a square steel plate about 6" square with the graduated rings maked on it and the 3/8" hole drilled in the middle. This was mounted with standoffs to a steel backing plate that had an old alarm system bell mounted side on behind the hole. When the pellet passed through the hole it rang the bell.
You could use a number of different bells, but stay clear of the cheap bicycle ones. A friend tried on of these and it lasted an evening before the bell was battered beyond recognition.
Hope this Helps,
Andy.