Widening deckover trailer

Hey guys, I would like to widen my deckover trailer so I can take my grapple and do three males across 9 feet. Right now there is a rub rail with stake pockets on the sides, any ideas for how you would widen this so I can also remove it? Last year I just screwed down three quarter inch plywood and removed it at the end of the season but it did not take the weight so well. Let me hear your ideas, thank you

Widening deckover trailer