For good sensitivity I'd recommend a specialised IC sensor.
The Atmel QT1010 is popular and very sensitive.
Adafruit do a breakout board with a 5 channel version, the QT1070 that looks ideal for you. It provides a corresponding output each time one of the 5 channels is pressed; but does not allow multi-touch (several at the same time).
Atmel's datasheet provides good info on how to make custom sensors that warns against the perceived wisdom of adding a ground plane, for example:
"Ground planes around and under the electrode and its SNSK trace will cause high Cx loading and destroy gain. The
possible signal-to-noise ratio benefits of ground area are more than negated by the decreased gain from the circuit,
and so ground areas around electrodes are discouraged. Metal areas near the electrode will reduce the field strength
and increase Cx loading and should be avoided, if possible
. Keep ground away from the electrodes and traces.
2.4.3 Decreasing Sensitivity
In some cases the QT1010 may be too sensitive. In this case gain can be easily lowered further by decreasing Cs.
2.4.4 Proximity Sensing
By increasing the sensitivity, the QT1010 can be used as a very effective proximity sensor, allowing the presence of
a nearby object (typically a hand) to be detected.
In this scenario, as the object being sensed is typically a
hand, very large electrode sizes can be used, which is
extremely effective in increasing the sensitivity of the detector. In this case, the value of Cs will also need to be
increased to ensure improved sensitivity, as mentioned in
. Note that, although this affects the
responsiveness of the sensor, it is less of an issue in proximity sensing applications; in such applications it is
necessary to detect simply the presence of a la
rge object, rather than a small, precise touch."