Hammering + Socket modd sounds fun, but it's relatively impractical when simply using a sharpie + quarter can get a perfect hole in less than a minute(if you are used to).
The diameter of the hole is 38cm, or approxmately 1.5 inches. I found a metal washer lying around that has exactly 38cm diameter. However, in order to get rid of the outer part of the pad from unnaturally bending outward from the tight fitting, you need to cut a second hole(48cm) over the factory cut hole + expanding the inner space between the front and back of the pad with a shaving tool or something appropriate. Otherwise, you would have a bit unnatural fitting on the grado frame, too tight.
The two other modds beside the first hole don't matter if the only thing you feel the senn pads are lacking is the absence of a hole, but if you are trying to replicate flats as my school alumnus and mentor vwap had done, they are worth the extra effort.
I think the only thing that can really mess up your modd is cutting the hole not Right in the middle. If you aren't too careful, the hole you just cut out might be off from the center by some milimeters.
For the type of music I listen to, I find the flats to be too bassy at times, so I prefer senn pads, and the added comfort is a plus
Try my Tripple-Bam technique.
Locate the center on the front.
Flip it upside down.
Bam!
Place them so the back faces up like in picture 2.
Bam!
Expand the inner gap so they can be seated on the grado less tightly.
Bam! eight slits. (Those black dots appearing next to the slits are ink marks that will disappear after dyeing)
Done.
Oh, and I think the only intended reason for the eight slits in the original flats was to keep them from tearing/damaging the pads when you take them out or when you are placing them onto the grados, given the hard/dense material. As you can tell, with the slits they are more flexible when you squish them, preventing wrinkles or tears. For the senn pads which is of softer and less denser foam, slits are just for cosmetics or for the satisfaction of many people who subconsciously compare them to the flats. If I were to ask vwap for his pads, I would ask for a pair with no slits.