Summary

I wanted a self contained touch plate. A CR20xx cell is pretty flat apart from the stamped ident (check by reflection, if it isn't the cell is blown and needs replacing) so gives power as well as the touch surface. You'll also need a conical spring from a battery holder, 3mm LED and 1-4.7K resistor (lower for brighter), wire and croc clip and m2x6 screws.
Straighten and bend the spring as shown. Bend the LED wires outwards using needle nose pliers so the LED fits onto the holder. Bend the wires round the holder with the flat on the LED closest to the edge of the base. This wire connects to the resistor and spring, the other to your flying lead. Solder up before forcing the straight bit of the spring into the base groove and juggle the wires to fit the LED on the holder.
Drop a cell into the box and fit the base with M2 screws. The distance between the cell surface and base is set to 10mm, all printed layers.