We're starting to get more and more request for outdoor signs. I've got the production process down, but I'm having issues finding the best way to mount signs flush to walls.

How are signs like this one normally mounted? Is there something like a french cleat that won't simply lift off? Or are they using an adhesive? I can't see much space if any off the wall.

icm_fullxfull.189488610_ajznxql5cyogos8kwgko.jpg