I think you have to ask why fit it like this when there has to be easier and better-looking methods of fixing a large sign panel like this?
It is a big panel at 4m x 1.5m
you wouldn’t fit a frame to a wall, then try sticking a panel of this size in one hit! I say that because can you imagine trying to get this level without touching the high bonding surface of the frame, then push apply it into place first time exactly level?? I just do not see the logic in that type of fixing, even though possible.
If like the picture appears to show, the panel has been fixed to this frame with no visible fixing points. I recon there must be some hidden brackets that are latching onto that frame and holding the panel. a bit like hanging a picture!
I am not saying this is the type of bracket used, "which I have attached" but something with a similar method could be on the face of that frame. the frame is fitted, then the panel lifted up and hung into the bracket slots with ease. then a few rivets to fix in place along the top. keep in mind the downward pressure of the sign slotted into a bracket will hold strong. keeper screws are then all that’s needed.
This is just a suggestion, as I really cannot see the point in having a sign hung like this.