My experience with clip-on fenders (specifically the Planet Bike model) has not been great. They work.... but not nearly as well as full fenders (I bought the Planet Bike Hardcore ATB).

I can't quite picture the problem you are having with a bolt. If it as simple as buying a nut/bolt as someone suggested.... seems like that would work! My PB Hardcore fender in front attaches to a hole in the top of the fork with a bolt; the rear with a zip tie. And then of course there are stays on both sides of the fenders...

-Jason