Originally Posted by
J.C. Koto
Yeah you can drill it, but unless you have a specific reason to use a particular reflector it would almost certainly be easier to buy a suitable one for your purpose off-the-shelf.
^^^^ This.
Except for most LBSs have a huge box of reflectors, taken off bikes whose new owners wouldn't be seen dead with a reflector on their bike. Mine let me have a couple for free. Get the ones that are typically used to mount on those plastic seatpost clamps. They already have a screw hole for mounting. Might as well get the screw too while you're there.
Then you just need to drill the fender. On my SKS P-series, the bracket for the lower rear fenderstay already had a hole, so it was simply a matter of extending it through the fender. A little Locktite on the screw and it's been there forever. All it cost me was a little time and a drop of Locktite.