The fingers did not go down thru the existing square holes. The red ink in photo below shows how much material had to be removed for these fingers to fit.
If you are reading this before cutting your square hole, then wait until this point to locate your hole. The plans and drawing have it too far forward.
I was able to enlarge the hole with a sanding drum and dremel tool. Easy, but I did scratch up the underlying angle. I'll sand that smooth and paint it later.
