Quick'n'dirty noise dampening mat installation in my trusty P180B. Bought a 50x100 cm piece of this mat from Motonet (cost 14 €, comes with glue). It's not gonna make a huge difference in the noise level, but I used it to block the excess holes of the case as well, so less dust will get in to the case. Took an hour to apply the stuff.
If you're gonna make this to your case then there are a couple of things you should take into consideration:
1. Only place the stuff to places where it doesn't make installation or getting the side panels in place hard/impossible
2. You don't need to expose the whole glue side, just cut away parts of the paper backing and it'll stick firm enough. This way you can also remove the mat in the future if you wish to.
3. A sharp knife and scissors are needed to easily cut the stuff. You can also use isopropyl alcohol or such to remove the glue from the blade (or just get multiple knifes).