Till now some of my installations run under Recalbox, but I will change all this with time.
The shader that I was always using globally is also present in Batocera. It just eliminate all pixels, but this should be to the taste of everybody on his own.
Apart from this, I use a shader for some Taito games that where original mirrored in cabinet and had no option to flip the game back to non-mirror mode.
This one is flip-horizontal.glsl and it is not existing in the basic set.