Thanks for all your kind replies.
Unfortunately, I don't have one security guard, but several. Applying a spray is out of the question.
I have made a kind of styrofoam hut to surround the cups. One of the walls is covered with some black flannel, to create some light/shadow effect on the piece.
I attach two screencaps for your appreciation. In one of them it's quite clear the "hut" that I talk about, although in this one there was no black cloth on one of the walls.
Any tips would be greatly appreciated since I'm very new at this, and have a great desire to learn.
Thank you in advance!