Just so people understand -- the function of the bitumen sheet lining is to reduce the tendency of the case panels to vibrate. It is panel damping. It does nothing for the airborne noise. So if you float mount HDDs and soft mount fans and generally use quiet components, there's really no audible difference between a case w/ and w/o this lining.
Lead surely would work well, but so would virtually any reasonably thick & heavy metal glued to the panels. The glue & the mass do the trick. And it's definitely not necessary to cover the whole panel -- I've experimented, and with adequate mass & a good glue, just 50% coverage is enough. Small sections don't need it at all (say any panel smaller than about 6" x 6", assuming it's not super flimsy thin) unless they're not secured, in which case they will flap around with vibration. Best to secure it rather than damp it.
PS -- the reason a small panel doesn't need damping is because it is much stiffer than a bigger one & unlikely to vibrate sympathetically at the frequencies where this becomes an issue -- ie, well below 1000Hz. It's resonances will be at higher frequencies, and it'll be harder for those resonances to be excited externally. And the reason such limited coverage still stops the vibration on a larger panel is because that's all the stuff is supposed to do -- stop vibration, not become soundproof. You can test this easily with any case side panel: With it off & the edges unsupported, hit it with something like a rubber mallet. Consider the vibration/noise. Now place the panel on your palm. Hit it anywhere. You'll hear a big reduction in noise/vibration. Of course, your palm is probably a way better dampener than the thin bitumen pads Fractal uses... tho I doubt you want to sit there with your palm on the side of the case whenever you're at your PC.
