If you don't wonna buy new cup, you can buy cheap glue gun,
then you put some glue on your cup, make the hole a little wet, so your glue want stick.
then wait few secs until glue will become a bit harder and close the cup.
Finally, after it will be hard enough, you can cut with razor glue you don't need.
And yeah, you can always warm up that glue with lighter, if you need to do step 2 again.
Works for me, even tested under water, but not for a long time though