I'm sure a local fabricator can do a better job and customize the size towards your personal needs.
Remember, that signs does not look as if it will blow in the wind, therefore making it somewhat stationary. If that's the case, according to the size of your sign, that back plate will never hold a sign 18" x 24" in a small wind, let alone under storm conditions. A much longer back plate and maybe even guy wires might be needed, but again, based on your size requirements.
Just food for thought.