An old-fashioned typewriter pad. They're like 1/2" of black closed-cell foam rubber, used
for a century (up to the Selectric era at least) to decouple the clanky beasts from
desktops. I once demonstrated their utility by hauling back & giving one a seriously
board-breaking karate blow, which resulted in a mere dull thud. Such pads can sometimes be
spotted in thrift-stores, & (now that I think) were sold in computer shops to put
under dot-matrix printers.
Barring that, sift through a hardware store. I'm
certain that 3M still makes the stuff in various blocks, as I bought a small box of 2"
self-adhesive squares last year from a surplus vendor.
And the same stuff is
used in workplace floor-mats, to relieve the strain of standing on concrete floors.
resident troublemaker, http://forum.frugalguitarist.com/