I have these headphones, they're my daily drivers, and I have tried different types of cups, different materials, thicknesses, and shapes. I can assure you it is a terrible mod to do, all it ever does is make the headphones sound narrow, closed off, and muffled. Trying to turn these headphones into closed cans is a terrible idea and will only make them sound worse when they already sound amazing. On a side note these headphones don't look that big but after looking in the mirror with them on they really are huge and cups will make you look ridiculous.
The only mod worth doing is changing the pads. Improved comfort, no soaking up sweat, more noticeable bass while being sightly more punchy, much more noticeable sub bass.
Here is my shameless promotion for my video on how to change the pads the correct way: