Mod your monitor so only you can see the display
This is genius. And like so many things that get that title it's also so simple it makes you say, "Why didn't I think of that?" This easy mod lets you turn a monitor into a private viewing screen that only you can see.
Essentially the process involves stripping the polarising layer from the screen then applying polar lenses to a pair of glasses. Since the polarising layer is what makes the screen visible to the human eye, nothing else will be seen by those without your glasses who will simply see a white screen.
So get yourself an old monitor and pair of 3D glasses, or any glasses really, then get cutting and pasting. Far more efficient than those dark privacy screen filters. Now to work out how to do it on our mobiles.