Unfortunately, on a flat DiBond sign attaching the panel clips is a little more difficult. I personally hate large signs with very little depth, so we would have made a panface out of either Dibond or aluminum for this job.
Another approach I've used on flat Dibond signs that large is to attach lengths of 1"x1" aluminum angle to the back on the top and bottom with lots of VHB and silicone. Then you treat the installation like you would a normal panface sign by mounting your opposing 1"x1" angle to the wall and hang the panel from those. As long the customer doesn't mind a standoff from the wall and you have room above and below the face to squeeze a few screws into the angle irons to hold the sign in place it works pretty well.
Another approach I've used on flat Dibond signs that large is to attach lengths of 1"x1" aluminum angle to the back on the top and bottom with lots of VHB and silicone. Then you treat the installation like you would a normal panface sign by mounting your opposing 1"x1" angle to the wall and hang the panel from those. As long the customer doesn't mind a standoff from the wall and you have room above and below the face to squeeze a few screws into the angle irons to hold the sign in place it works pretty well.