No idea why this did not occur to me earlier, but Rivnuts/Nutserts/Plusnuts are a decent option for mounting to the manufacturer's 2x2 aluminum cross bars. So I think I'm going to end up ordering four cross bars and using standard issue z-brackets and angle brackets to mount the panels in pairs parallel to the axis of the trailer. Angle brackets on the outboard side and z-brackets on the inboard/middle sections. I really need to draw this.