I have done a number of mods to buckling springs.
The floss mod works great, it cuts down perhaps 1/3 (maybe almost half) of the noise in actual decibels, but takes away the "ringing" component that carries. So, while it may not seem all that different to someone in the same room, to someone in the next room it makes all the difference in the world.
Padding inside the case is difficult, because the clearance at the bottom (spacebar) is very tight and you can't get much in. Felt is probably your best bet down there. It does help some.
Setting the board on a nice pad on the desk can be a big help, I use the cheap "waffle" rubber drawer liner from Wal-Mart.
My primary board today is a big ole Model F 122-key terminal with floss, felt, and some extra pillow stuffing packed loosely inside, and set on a drawer liner rectangle on the desk.
The feel is good, and the family does not complain because the sound does not carry nearly as far. When I get out my nearly identical, but not flossed and padded F-122, the difference is striking.
I have not tried the grease mod, it is supposed to work, but sounds gross and difficult. Also, the floss can be pulled out quickly if you don't like it.