Yea I was thinking that too after being unsuccessful finding something that I could just stick on top of the pads. There's actually plenty of room beneath the pads, not very tight at all so you could actually make it stick out quite a lot this way too. What "foam" did you use, I'm having some hard time finding something worth sticking under them, may have to use toiletpaper lol.
EDIT: Yes it worked nicely! I definitely approve this as a very easy soundstage enhancing mod. Just need a piece of papertowel for each side as that will have the perfect length that you roll into like aprox 1cm wide paper tube/roll and stuff inside:
EDIT2: It might actually have increased the bass response a bit, that's what my early impressions tells me. O.o I would be interested in hearing opinions on this mod from velour pad users.
EDIT3: Seems to provide a more V-shaped frequency response, the midrange seem to step backwards a bit like this.
EDIT4: Make sure at the upper-edge it's tighted very further back inside as otherwise I think you might be covering the upper vents which seems to result in a brighter/thinner sound and less airy soundstage. I seemed to have done so at first, started sounding better when pushing it further back inside the cup (mids come more forward again).
EDIT5: I don't seem to need any EQing anymore after doing this mod. I applied a slight V-curve EQ shape to it before which I don't need to with this mod.