No real need to go any higher than the frame you have on the trailer now. You are just making a larger sail to give you wind resistance and lower gas milage. Other than bugs, any road debris will be hitting low on the trailer. No need for side panels.
I have used my snowmobile trailer to haul my bikes when I have moved. At least 7,000 miles and no damage to the bikes with the lower shield. When I used it for snowmobiles in the winter, the slush was always on the lower section of the shield.
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