Reattach Tub Overflow

OK, during the process of clearing out a severe clog, I undid the overflow cover to get a snake in -- and whatever it was that connected the durn thing to the fiberglass tub crumbled and fell away into inaccessible-land. Now there's a 1/2" gap between the familiar horn-type opening to the pipe and the tub wall.
I need some sort of fitting that will bridge this gap, but I haven't the slightest idea what to look for -- and presumably bonding with fiberglass is something that needs to be considered as well (will regular plumber's putty work? I have that).
Is there something that fits INTO the opening on the pipe side and snugly fits around the opening on the tub side? That would seem to be the ticket.
Is it possible there was no fitting, and just some sort of putty-type concoction, to begin with?
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Dan Hartung wrote:

There is a gasket between the o'flow and the back of the tub. It may have been a thick rubber foam type. Common item at a hardware/supply house, but there are a variety of types.
Jim
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