If you can find a rubber cap big enough, it will work.
In a pinch a disk of plywood with a washer and a regular nut will work also.
It don't need to seal completely, just block most the flow.
If you want it to seal, cut a grove round the outside of the plywood disk, and put a bead of silicone in it.
If you get creative, be sure to post pics.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization