I had a similar problem a while back with plastic fenders. You may be able to cut away some material at the fender's brake bridge mounting bracket to buy a few mm where clearance is tightest (esp if the bracket is plastic). Or purchase a stamped metal bracket like the ones offered by Velo Orange, cut a slit in the fender (round the edges) at the appropriate location, and mount the bracket inside the fender with the brake bridge 'mounting ear' coming through the slit. Use a good epoxy or adhesive to secure the mounting bracket to the inside of the fender - perhaps a rubbers cement or adhesive with some give to it once dry.