As panels are most efficient when pointing directly at the sun, I would be inclined to hinge mount it on the front rail and have an adjustable bracket on the back rail. This is so you can leave the panel attached to the trailer and tilt the panel so that it gets sunlight hitting it at as close to 90° as you can.
Then as the day passes, you rotate the trailer drawbar to point to the sun.
The reason I would home on the front rail, it's in case the the other side comes loose, the wind form forward motion isn't going to get under it and flip it into the road.
You could even get clever and make a "sun sight". A + slit in a top plate with a white + painted on a back plane. Then when the panel is pointed directly at the sun, the sunlight passing through the slug slit will shining on the white +.
Alternate method is a thick plate with the slit and when light shines through it must be pointed at the sun.
Also carry a compass for finding north on an overcast day. You never know when it might clear. If the panels already facing north when it does, you start producing the most energy straight away.
Cheers,
Gary
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