Adventure trailers sells a higher mounted rack as well.
Really there are tons of options, just depends what you're looking for.
I built a basic shelf with painted/bed lined wood and c channel aluminum, it sits on top of the rail of the top. Room for my cooler, water containers, chairs etc below it.
Eventually I'll probably buy the shelf from adventure trailers to allow mounting an arb fridge on a slide below it.
Here's the one I threw together.