The director is a mamet purist. The script says lunch BAG, not BOX, so the milk is inside the lunch bag is inside the briefcase rolling around until the character opens it and pulls out the milk. The reason we are using a carton is for the time period, that the most common way he would have brought milk would be in a carton, not in a thermos or bottle with cap or other much easier method of dealing. And the milk gets drunk onstage every nite. AND, if you've ever tried to open one of those things as an adult, esp as a senior (who has to open it) it's trickier than you remember.
I would love to rig something inside the briefcase but the milk has to lay on its side, whatever is holding it closed gets wet and then, well, we leak.
I would love a pull-top bottle, or a screw-lid, or something I can manipulate, but the director wants this. And our job is to say yes until we can prove it can't be done. So I love all these suggestions, maybe one of them will spark something that will solve this dilemma before we see audience.
Thank you all - keep sending ideas!