Your adversary is extremely simple. You haven't mentioned in your post if you are open to or able to do any custom coding. I would suggest writing/getting an

Your adversary is extremely simple. You haven't mentioned in your post if you are open to or able to do any custom coding. I would suggest writing/getting an app written that runs in fullscreen and looks exactly like an OSX login screen. Bonus points if it can disable multi-tasking shortcuts such as the 3 finger swipe up. The app can be hardcoded to only accept one password - yours. Since the app is running with OSX logged on you can use your usual tools to enter the password. It goes without saying that this won't fool anyone determined - you can just reboot the laptop to make the app go away. However it should be enough to stop casual passers-by.