If it is the rear, then get a longer bolt and a nut that will fit it. Thread the bolt through as far as it will go, then attach the nut the the end and tighten it.
Just a thought though, when putting the bolts into the rear fender, put a bit of grease on the threads on the bolt. This will stop crud from sticking to the exposed thread insid the wheel area. Also don't tighten any of the bolts untill all of them are in place and threaded a decent way in - then tighten them.