We have been doing this for years....the trick is to buy the glasses first then match the holes so the glasses just press fit in. They will still pop out but when you do your "shots" you hold the bottom of the glass that is popping through the bottom of the ski. Then when you tip it over there is no way you will have it fall out since your holding it and the ski wont slide out and pop you in the face if you use a tapered shot glass.
Good times we almost sunk the boat on that one!
JTR