I've got the same problem with a bike I just ordered fenders for. My solution was to order some of these in the appropriate sizes. Fenders and clamp/mounts are making their way here in the mail, so I can't attest to how effective either turned out to be quite yet.

If you'd like, I'll post up the results here in a week or so. Unfortunately, the fenders arrive tomorrow while my clamps arrive late next week.