I made a frame for my signs with ¾ iron channel and cut a piece of ½ PT plywood to go behind the sign for strength. I then placed plexy glass over the sign, all of this fit inside the channel. I attached the frame to the back gate with 1½ angle welding the angle to the angle of the gate. Then welded the frames to that newly attach 1½ angle. I used the plywood to keep the wind from messing up the sign when driving 60-70 mph and provide additional strength. I welded the side signs on the wire box up front. I cut the corners at 45's and welded top, bottom and one side together making like a horse shoe, I then cut small pieces on angle iron the same width as the channel and welded them to the side I didn't weld together yet. I then drilled a hole through the angle iron into the channel of the horse shoe piece and tapped the channel so I could used a screw to attach the forth side of my frame where I could remove that side piece of channel where I could remove the sign if needed. I would show you a pic but don't have a dig cam. any longer.
Hope this helps,
P.S. Drill 3 or 4 weep holes in the bottom of the channel to allow the water to drain out and make sure you primer and paint after you drill the weep holes.
