I am BY NO MEANS an experienced builder-but I am by nature a desperate scrounger....
A piece of strap steel (roughly as ductile as the ones that fasten brake arms on the Coaster rims, or just a little harder) will work wonders. Bend two strips of steel 90 degrees and attach to the fender (the easiest method would be to just drill a hole and screw or rivet-a good Industrial or Steampunk look) directly next to each other. Then, measure the distance to your rear fork (and an extra two inches or so) separate and bend each strap on each side of the fork shown in the picture above, drill holes on the straps and use some tiny bolts to hold the straps in place "coaster brake" style. Scratching is unlikely given that fenders are stationary, but you can add some tiny bike-tube gaskets if you want...and the dual straps and bolts will help with that Early Industrial look .
Otherwise you can bike the bullet, drill one hole in the rear fork and attach a simple "L" bracket-but why use the pedestrian Box Store method?
