Don't know where you are in Pennsylvania, but around here, if you flush mount a sign to a wall, there's generally no wind load/sheer involved.
There are several correct ways to mount a sign to a wall. If you're gonna use thin 1/8" stuff, ya need to build a frame from either wood or metal, preferably aluminum. Mount the sign to that. If you use thicker material, you can use french cleats... or a frame. Depends on what the customers wants. Some customers don't wanna see any fasteners showing, so you might build a top and bottom holder, bend the lip and screw down from the top and up on the bottom and the same to the sides. Stand offs for something as flimsy as 1/8" I doubt would work. Not saying it won't, but doesn't sound feasible.