Here are some thoughts. How much does this bad boy cost?  Would it be cheaper to buy a utility trailer and add the bars as a diy Project? One could add treated 2x4s painted with marine varnish.

Check the 'wear factor' on the canvas cover. You might want to add some support bars over the box. I would glue pipe insulation or pool noodles to all four top edges of the box.

Just my nickles worth. 8-)