The picture below illustrates how I carefully lay a protective cloth over the parts mounted to the "circuit board" in an EM game.  I do this to keep parts that I may accidentally drop from the underside of the playfield from disappearing into the switch stacks or Score Motor.  If you do any soldering it will also keep solder from dripping onto a part that could short something out.  My cloth liner is an old waterbed liner that has been cut to fit in the bottom of the game box.

(click any picture to enlarge)