The cover shown in your link should be perfect for your application.
It is impossible to see in the photos of my trailer posted earlier, but the Canback top I had custom made for my Bantam is designed to bolt to the flat top rails on a standard pickup bed. Since these 1/4 ton trailers all have a tubular top rail, I had to fabricate a flat surface. My solution was to use 3" flat stock with short lengths of 2x3 rectangular tubing welded underneath to slide into the stake pockets in my trailer. the flat stock rests on the tubular top rail and a bolt in each stake pocket holds everything in place. The entire assembly can be removed to return the trailer to bone stock.
Since you don't have stake pockets, your best solution is probably angle stock over the tubular top rail bolted to side of the trailer tub. Since angle stock is available in many dimensions, you can probably find something that will exactly match the width of your tonneau cover. If the width is a bit off, I'm sure there is a shop in your area that could cut it down to the size you need for just a few $.