I have fitted a roofbox to my little erde 101 , I had a roofbox that I got off preloved for £30.0 but had never fitted to my car any way I got to thinking how to fit it to my trailer and I came up with this, I bought some 1 inch square tube and cut 8 bits 3 inches long welded them side by side in pairs bolted these to the sides of the trailer with some bits of plate on the inside. Then welded a piece of rod into the ends of 4 bits of tube 9 inches long dropped those into the bits on the trailer then two last pieces of tube across the tops and bingo! I have also drilled a hole through the bottoms of the rods to put an "R" clip in to stop it from jumping up when traveling.