Jesse at the Gasbox made these cast steel, weld-on rear fender mount brackets, meant for welding to the seat cross bar on rigid frames. Cut, grind, file to make it your own. You can rework the radius that fits against your fender or flatten it out to fit the profile of the fender you are using.
Each custom after-market frame being different in geometry, the seat cross member may be closer or further away than you need. So you may need to cut this bracket down or if necessary have to move your seat cross member. Made in Cleveland, Ohio. At the Gasbox.