Simplest touch sensor for a copper surface
Split the touch area...

Split your copper touch area into two parts that are interdigitated. Then it's very easy. If the user touches the pads there will be a current path between the halves. Otherwise not. The resistance can be quite high for a "dry" finger (~10k Ohm), so use an amplifier to ensure reliable operation. A simple single-transistor type is sufficient.

...like one of these!