A tub drain that fits a 2.5" drain opening?

I'm replacing the drain system on an older cast iron bathtub, and the drain opening at the bottom is 2.5" in diameter, which leaves a lot of extra room when a standard 1.5"drain is put in. The rubber washer at the bottom barely has any overlap with the edges of the opening, so I'm afraid it's not going to make a very good seal. Nobody seems to stock or even be able to special order any larger drain sizes.

Any suggestions?

drain

It could have been one of the "ancient" Kohler tubs, but they no longer make the oversized drain for theirs.

Thanks. I'm finding that they are impossible to find. What would you do in that situation?

I think Watco and/or CS&B makes a bushing