Get a small tube -- or is it called a pipe? -- about 1 cm (1/2"-ish) outer diameter, 1 or 2mm wall thickness.
Bend the tube to make it fit over your tire.
Measure required length (desired mudguard height) and cut the tube to size.
Drill holes to mount the mudguard at the top and for the caliper mount on the bottom.
Paint, assemble, enjoy.
My DNM USD-8S fork has double brake caliper mounts, one on each side, but no doubt this can be squeezed under a "in-use" brake mount as well.
Please dont mind the giant washers on top, those got changed later.
And NO, you cannot just cut and upside-down U from a 1mm sheet metal... Trust me I tried, it bends. Also the tube is faaaar less work...
EDIT: Mods, please, if possible move this thread to "technical" section