That's a pretty good source of fender material. I'd add a few small things to keep the fender from being floppy. First, I'd use a longer mounting angle so it spans to the front of the fender. Then the smaller stiffener angles could be tied to it. The metal between the large and small angles is where I'd expect it to crack. Maybe add a third stiffener going straight up. It would be good to have something on or near the top if the fender with a bracket on the ROPS to hold the top of the fender.
When we replace the fenders on our utility trailers we caulk all the joints with automotive seam sealer. That keeps the moisture out of the nooks and crannies and helps them last longer.
