This is a very simple construction that will enable you to mount mudguards/fenders or racks to any bicycle that doesn't have eyelets for screws.
Step 1: Materials/Tools
10mm Ply wood
Hole cutter bit 20mm
Small drill bits
Step 2: Process
Cut hole with hole cutter bit. Use the "waste" circle, discard the hole.
Saw into two semi-circles.
Drill two small holes in each for cable ties.
Drill holes for fender screws. Take care to make these smaller than the screws so that the screw thread can bite. If the screw is tight, there will be no need for a bolt.
A nice touch is to file a concave profile on the flat edge of the semi-circle. This can help the mounts seat on the bike forks or seat stays. I have tried rubber shims, cut from inner tubes, to seat these mounts but they will sit perfectly well without. Just ensure the cable ties are tight.
Sand and varnish to waterproof and finish. Epoxy or even diluted PVA will work too. You could of course paint to match the bike but then people might miss your genius.