i used a peice of solid steel i think 2.5inch round stock.bored a hole and tapped it.drilled a hole just outside u-bolt plate and bolted it on.then i tacked a bumpstop to the frame rail. too a try or 2 since i didnt have a way to flex it really.
marlins instructions i beleive tell you to tack a bumpstop to the plate and weld a peice of metel to frame.
mine works very well now that i got it setup right.