The little hooks are perfect to hook up the elastic cord or elastic rope, to fasten a tarp or net over your trailer to secure and protect your load on your standard [Masta trailer](https://masta.nl/enkelasser-257x131cm-750kg.html). Make sure that you print with 100% infill, a hollow print would be to weak for the task.
I usually design things in Sketchup to print stuff on my UM1 or UM2. The designs are always accompanied by their source files for editing and a collada file for printing but if you want a particular format, just ask.