I have seen several threads about how to cut the notch in the WSM's and pictures of it but I decided I would do a video of actually doing it.
I used a Dewalt angle grinder with a 1/4 inch grinding wheel. I didn't use any tape and just eyeballed the depth of the cut, I made it so it just cut the shoulder of the rim. It is a very easy mod and took about 30 seconds for each cut.
Please check out the video