Sorry you're out of luck, you could get one made from a local canvas shop as you have the old one for a template but ( advice with a little bit of knowledge) you'd be paying thousands not hundreds, I'd take a stab at a complete one with all walls etc close to $4,000 retail.
you could make a cover just for the trailer without the extensions up or out, it'd look the part but probably still $1,000. Someone on here did that, maybe Juddy on his workshop (the tray on the Land Rover workshop vehicle and workshop trailer are identical)
hope that helps
cheers
Scott