We never found the need to use any sort of screen in the RAF - occasionally the odd student had to be made to wear a visor if he tried to cheat. However, a bit of 'Chinese weaving' in solid cloud on limited panel soon stopped that. IREs know darn well when someone is cheating as the recovery to datums is different when flown visually!! But this discussion on screens means that I will NOT, under ANY circumstances, fit such things to any of my training aircraft - foggles work just fine!!