Nylon tie might work if you have a mechanical tightener for the ties. Hand tightened
would be problematic (noisy). I assume the fender attaches in the front to the
bridge between the chainstays just behind the BB and via a wire bridge with formed
eyelets to the frame eyelets with a bolt. So is the problem the second wire bridge
with no attach points? Not long enough to reach the existing eyelets? One possibility
would be a piece of 1/16 or so al strap say 0.5" wide and long enough to reach the
other wire bridge from the fender. Drill hole in one end big enough for the bolt through
the eyelet to go through, bolt it on there and run it forward for the other eyelet to
attach to another hole. It might have to be formed a little to go around the seat stay
but thin al is easily bent. Steve