Ever lost one of the caps on your contact lens case while on holidays? It's annoying, but instead of rushing out to buy a new case, Redditor westleyh suggests just using the cap off a 600mL bottle.
Most contact lens cases are a standard size, and that means they can use caps from all sorts of things. A 600mL bottle cap worked great on a couple of cases I have. Just take the cap off your drink and screw it onto the contact case just like you would the cap. It didn't work with all my cases, but if you're in a pinch with a lost cap, it's worth a shot.