The air pump leather is cup shaped. I bought this pump leather many years ago from an elderly gentleman living in central Illinois who was making them for Model A air pumps. It fits well and works well. . Sorry , I have lost his contact info. Perhaps a supply company that supplies pump leathers for old water pumps. etc? In order to pump air, the leather needs to be cup shaped like this.
The rubber cup used in a hydraulic car brake system Would be ideal if you can find that diameter or 1 within maybe 1⁄16 of an " of the actual diameter you might try searching the internet for that.