Haven't there been about 50 of these threads???
I never use public restrooms, unless I can absolutely help it.
That said, LOCK THE STALL!!!!! If you're so worried about the Zombie Attack that you sit there and fret about whether one of the Flesh Eaters is going to reach in under the stall and grab your gun, then HOW in the WORLD do you expect to relax enough to go to the restroom! If it's really "eating you" so much, AFTER you lock the stall, take the thing out and set it on the back of the toilet seat, stick it in your boot, swallow it, or if your stall has a coat hook then hang it by the trigger guard. Then, sit on the john and pray the Zombies do not hear you.
Really, I understand that some folks just sit and think about this sort of thing, but this question (repeated as many times as it has been) is absolutely ABSURD!