When you get back home, you've done it! Now as a reward, I want you to finish the job you started in the changing room! Get yourself off, you definitely earned it!
Think about all the people that saw you today while you do the job. What do you think they told their friends? Or even better, what do you think they did once they got back to their house, or hotel, or whatever? Do you think they're doing the same thing you are right now, and thinking of you?
If you're really getting turned on by the exhibitionist side of yourself, then move yourself over to a window while you get off. Or better yet, outside! How fast do you think you'd cum if you were doing this on your hotel balcony right now?