It might be as simple as drilling a small hole, slightly smaller than airline tubing, at the very bottom of the cup and forcing a piece of airline tubing in and letting the pressure from the fit make a seal. . . I think as long as the hole is drilled fairly well I think this could work but I can't guarantee a seal haha
Other options might be to take some acrylic tubing, slightly larger than airline tubing, and drilling a whole and "welding" the acrylic tube into the hole, making a numb to attach airline tubing too.