Another thing you can try is to buy 2 magnets of decent strength, glue one to the bottom of the keypad and place the other on the underside of the table. (these have a 6.1 lb pulling force and should be small enough for your project, but do be careful, neodymium magnets are very fragile especially thin N48's) Then, place a strip of rubberized tape over the underside of the keypad including the face of the glued magnet. Because the Micro-controller uses flash memory rather than magnetic disks, the presence of the magnet will not erase any data. The rubber tape plays two parts in this, it increases the friction with the table surface, as well as damping the vibrations caused by key-hits. (which keeps the board from being vibrated off position)