The way I do it is drill a 3/4" or 1" hole (large enough for the saw blade to fit) in the corner so that you can insert the saw blade and you should be able to get you opening cutout without any problem.
Here is a link to a YouTube video () you can see what I am talking about at the 4:34 mark.
BTW I have no affiliation or even know anything about the young man in the video, it is just one I googled real quick.
Hope that helps ;) !LUV