OK, what you drew would work. Computer case should equal ground (even with some paint on it) but not real important. The 555 circuit I provided should work with that. That or as I mentioned make a pad. Just find some board material that is copper clad and cut out a circular piece. Then file away the copper and make a slot down the center. You now have two conductive halves. Run a short twisted pair and connect to each side of the slotted pad. Placing a finger on the pad will bridge the gap and that should do it.
Ron