Put aside what material to use for a moment.... will anyone really be able to see a
sign, that far back from the face of the glass from any kind of distance ?? In other words, other than someone walking past, do you think drivers will be able to see through the glass that far back ??
Do yourself and your customer a favor and put a fake
sign in the window back about where your proposed
sign will go and do a little experiment. While driving by, look for two or three seconds and see if you can read anything without having to study it.