I'm going to flip people off through the door when they come try to open the bathroom stall while I'm pooping and they realize its locked
Why would you do that to someone?
I don't like people who can't respect my space.
I think just saying the stall is occupied is a far better response to the situation than what you're describing.  You shouldn't be rude like that to other people in social situations like that, especially when it's an accident on their part.
It is a hassle to do all of that. I think my way is more efficient.
It is expected that you be polite in public and flipping people off would not be called for.