If you want to add some heavy metal to the bottom of your keyboard, why not use Osmium?
It's real. It's the rarest non-radioactive metal on earth, and the most dense (i.e., heaviest per cubic cm.)
The only problem is that its pyrophoric. Meaning it spontaneously combusts if it comes into contact with oxygen at room temperature. So you will have to seal your HHKB up air tight, but otherwise, a few short and thin bars of osmium will weigh that HHKB down good. Oh yeah, osmium also combusts in water too, so don't get it wet.
The only other heavy metal I can think of that is dense enough to work is Platinum. But if you have that much platinum laying around your house, you can afford to hire somebody to simply hold the HHKB in place as you type, so just do that instead.