I don’t have a dually so I can’t opine on that issue. My Bundutec fits right up to the headache rack on my flatbed and there is a small strip of plastic that acts like a bumper.
I would think you could easily fashion bumpers out of wood to close that gap. Use salt treated or paint them to help them last. A 2X6 is really 1.5″ thick so 3 of those plus a 1×6 which is really 3/4″ thick would make up that gap nicely.
Of course, given current lumber prices, the extruded ones might be cheaper. 😉 If it were me, I’d use the wood as it would me a full width bumper that could easily be replaced if needed.