Hi Tom,
I've also looked around for a proper bell target and drew a blank.
I have found faceplates, bell and 25mm spinner online and plan to build my own.
The design will have the spinner behind the 3/8" hole taking the pellet impact and acting as the striker on the bell.
To further signal a 5 score I will drill a small hole in the bottom of the spinner and have light weight line (fishing line) attached with feather or similar hanging below the target. Movement of the spinner will be signalled by the indicator.
I am sure others have come up with better ideas over the years?