Trying to secure the trailer lighting board on the trailer.

I've been working on the trailer board support arms, to see if I can come up with something which will actually hold the board on. Originally there were steel brackets on each arm. One just seemed to have fallen off. The other is so sharp edged it chafed through a bungy cord. So I have drilled both end plates and bolted on a marine ply upright, with a lip to take the weight of the board, and a bungy which is only in contact with wood, not steel. Hopefully this might work. I have little faith in trailers. I have found that I have to order the board specially because no local trailer firm carries 6ft boards in stock.
When the boat is on the trailer, there is no problem, the board is supported on the boom gallows on the boat, where it is secure, at the back and very clear. I am sure that a board underneath the boat is just about invisible in practice.