you are wanting to put a drain fitting on it so you can drain it into another container that is bigger? i'd put the drain on the bottom of the cup where it is flat and where it is lowest point. you might look into the airline tubing accessories or maybe RO/DI fittings section of whichever parts company you look at...
my
coralife skimmer is already made with this feature, with an attachment for airline tubing in the bottom of the cup. it has a plug for when you don't want to use the feature. i have the largest model. i don't use this feature because the performance is enhanced by cleaning the cup every few days, so i might as well empty the cup. it might come in handy when on vacation, though...