I purchases the fiberglass cover for an aluminum utility trailer. I wanted the trailer to last for decades so thought in Northern Calif. having a cover to protect it from sun and rain would be a good idea. The cover fit well and has straps to tighten it. Their is an opening in the back, so it is easy to unzip and lift the back of the cover to get into the trailer when I need to.
It also has a back flap area if your trailer has a back shelf to hold tanks or other equipment. It also has vents, so even though they say not to use on aluminum trailers, it is working fine on mine.