I am looking for a source for aluminum inspection hole covers and
backing rings for metal wings. I could make them, but would be nice to
find some ready to install.

You could always order them from a manufacturer. Cessna allows you to add
inspection holes and covers when you need access to some part of the wing not
reachable from existing holes. I suppose Aircraft Spruce or one of the other
suppliers might have ready made ones as well?

John Dupre'