We have the use of a trailer, but this trailer is a little too small (2 x 1 m) to transport full size panels (usually 2.44 x 1.22 m or 2.5 x 1.25 m).
So I devised this rack which can be put on top of the trailer, and attached with ratchet straps.
The straps also keep the load down on the rack.
First the three top cross bars are put on the trailer. Followed by the two long ones. And finally the last cross bar is put on the long ones.
To make sure the last cross bar cannot "jump" out of the long bars, I drilled a hole diagonally through both and put a homemade bolt through.
The short straps go between the two middle thick cross bar sandwiches, and around the top rail of the trailer. This keeps the rack down.
The long strap goes over the whole thing, and around one of the rails at the head of the trailer. This keeps the rack from sliding off.