The device is called a "third eye chain watcher."

I also have problems with cross-chaining. On my mountain bike its caused some problematic chain suck I could not fix (thus I now have a third eye chain watcher). So my goal is to get better about shifting when I am suppose to shift. On the mountain its easy cause I can watch the shifters. For a road bike a Flightdeck computer is a must or have a friend ride behind you and watch and remind you to shift. The fear of getting yelled at is worse than the cross-chaining for me.