I fitted mine without drilling using a high grip silicon. Works a treat. Did the same on my tiger 800 which had the extender on for 10 years in the same fashion without issue.
I thought about that too. I just don`t have a glue ( ) what kind a glue should use and the selection is probably different in every country. Of course mechanical attachment would be nice safety backup but no way I´m going to drill the stock fender. Worst thing that could happen is that the extension drops under the rear tire and just flews away. I don`t think it could cause lost of control or anything.