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I am assuming your trailer gate is the fold-down, with expanded mesh type, typical in most trailers.....

on my rig - Isuzu NPR with a landscaper body (flatbed, with a 4' sloped ramp section, with a fold down gate), I mounted a metal aluminum 4'x6' large sign on the back.  I used pop rivets all around and some in the middle of the sign.......  note, a 4'x6' piece of metal in the wind stream has A LOT of wind resistance - thats a big surface!!!

I THOUGHT I had engineered the fastners well enough the first time with the rivets, but the first time out at 50mph, some of the rivets tore loose... If I would have kept going at that speed for any longer, it would have ripped off completely... I was lucky I caught it in time...

I went back, removed the rivets, and replaced with hex-head, self tapping metal screws - the ones with the black rubber washer attached.  Went all the way around the perimeter of the sign, every 4 inches, and "in the field" of the sign vertically too on the vertical supports of the ramp.

I suggest OVER engineering the fastners, or you may LOSE your sign to the wind resistance!!!!

I've always used Alumalite sign board. It's a coroplast sign with aluminum panels front and back. This makes it much more rigid than a standard aluminum sign panel, which tend to crack and pull apart from the wind force. To attatch, you can use self tapping screws, drill and tap threads for each screw, or just drill through and use a bolt and nut.

zip ties and self tappers