To provide my input on support in the middle to support the weight, I agree you will need it. Driving along these could start bouncing.
I suggest a simple block of wood or two. Sand and round all edges. Cover in felt (or other material suitable for outdoors that shouldn't mold).
Use velcro on the top of the block to the panel to "keep in place". Also, with a little tension (the block actually supporting middle of the panel), it will keep it in place.
This is just my 2 cents worth of input -- using simple wood. It's what I would try to use/do.