One thing I might do is to let everybody know that you are using a straight, show it to them & maybe demonstrate on a bit of arm hair. Then explain that you're going to put a sock on the knob when you go to shave and that knocks on the door could end in a sliced up face with blood everywhere and possibly an ambulance ride and stitches. Not that it is that dangerous, but they don't have to know that.

Then, take your everloving time while they contemplate if they should knock or not once they see the sock...