Hi Jamie!
Same here, I got an XLR cable from FAW (Forza Audioworks).
Did crosslistening between my unbalanced modified HD580s and balanced unmodified HD800s.
Cymbals and high notes around 6kHz were indeed very obvious and fatiguing on the HD800s doing the comparison.
Doing the mod, I stuck to 2 mm of foam(Creatology like material) and 2mm of polyester felt on top of that with doublesided sticky sheet in between, just like in the video and now it is really fantastic. The shape of the cutout fit very nicely over the metal ring and the back wall of the Phones.
I used black foam and felt and it looks like they came shipped like this.
You won't regret this.