I know it may sound weird, but those Water bottles with the softer plastic screw on tops such as.. uh oh forgot what they are called, they have a white screw on top, no spout, and their picture has like a mountain, a lake maybe, i think the name is two words... crystal something... im not sure. But ANYWAY, try taking a pen and piercing a hole or two in the top. Sitting on your desk it looks like a normal water bottle and when you drink out of it it looks normal unless someone is really close to you. You may have to suck kinda hard out of it... 3 holes is best and you can't really see the holes just glancing at it.