With some of the crud yu see n a collection cup, air line is oging to be very quick to clog up a drain line. Best bet is if your dead set on adding a drain line is to go to a big box store and get a fitting that is called a hose barb to male adapter. It will be on a rack in little plastic bags in Home Depot or in the rack of drawers in Lowes. I would look for at least a 1/4 male pipe x 1/4 hose barb. Then pickup a length of the precut 1/4" clear vinyl hose. 1/4" is about the smallest diam I would fool with and feel comfortable with. Drill the cup or dremel out a hole to fit adapter male pipe threads, let it extend into the cup, and use Marine Goop (not silicone) to seal and adhere it in place. In about 12 hours it will be dry enough to put back into service as long as yu do not put any stress on the adpater thats glued in place, so its a good idea to plut tubeiing on adapter before sealing it to cup and set aside to dry. Marine Goop sticks pretty darn good to the materials normally found in use with aquarium equipment etc. All parts canbe obtained at most any big box builder supply. I would look for a nylon or PP or PE adapter. MOre than likely they will be white in color. YOU can also find 1/4" double ended hose barbs which have a flange midway on the fitting that will also work just fine for adapting a tube to the cup. You can always cut off one end of the fittings hose barb portion to reduce length of fitting inside of cup if needed.