Some additional thoughts after studying this pic. Using the same half round strips to simulate the corregated sides, what if we take a trailer kit from Galaxie Limited as a base. Then make modifications from there? Would be sturdier as a start point, would come with the back gate, easy to get various versions from them direct, ebay or other suppliers
http://galaxielimited.com/trailers.htm
http://www.modelroundup.com/v/vspfiles/photos/Galaxy-T-38-2T.jpg
http://www.ebay.com/itm/Galaxie-21-Foot-Tag-Along-Trailer-Kit-1-24-or-1-25-scale-HOBBY-TIME-MODEL-SHOP-/221456389324?pt=Model_Kit_US&hash=item338fd454cc#ht_739wt_662