Marc, take a close look at how I mounted mine. At first I also thought I had to drill the fender but it's not necessary. On the side with disc brake tabs, mount the attachment bracket on the disc brake tabs. Then on the drive side mount that one slightly higher so as to clear the chain. Mine has a few mm clearance and I also use 10 speed cassette. This means the attachment brackets are slightly asymmetrical. It's not noticeable just looking at the bike. Having them asymmetrical does not adversely affect the adjustment of the fenders. My guess is you tried to place them symmetrically at first, like me, and faced problems with chain interference -- like me!