I’ve often thought about how to fit a decent off-road mudguard in that position so that it does a decent job, just like my trials motorcycle.
I havent tried it yet, and it should stand commuting, but fill the middle recess on the back of the brace with say metal putty (it won’t stick enough to hold it adequately but it’s only to prevent movement). A small metal plate, cut to match the curve, goes over that and the mudguard bolts through its own bracket to the nut (which could be welded to the plate). The plate is held to the brace by colour matched plastic ties. If I could put a sketch on here I would.
Worth a try?