We tried putting the cap in hot water for a while but still could not get it to fit.

Lid fits very tight (be sure to follow directions and boil it) but I don't trust water inside my bassoon case

Well sealed, doesn't leak water

First, you can’t quite see in the picture, but the cup has markings on it for the volume of water, with markings at 30 mL, 45 mL, and 60 mL, as well as at 1 oz, 1.5 oz, and 2 oz