Aluminum angle or c channel is how we have done many like this. Stick a couple of pieces to the back of the
sign with VHB tape. Attach two more pieces to the wall. This crude quickie drawing shows what I mean.... Of course size will dictate certain things. For a very big
sign you might need some center bracing or more surface area of the aluminum angle stuck to the back of the
sign with VHB tape. Also need to make sure you are using the right type of high bond tape for the job, there are a lot of different types for different uses....