you could simply rectify what 60Hz energy is coupled into a tapering metal stripe udre som ethin insulation. By peak_detecting the energy, you have a linear measure of capacitance; this will change dramatically, as your body moves around in the Electric Field in your workspace; an alternative is to rest your forearm on a plate driven by 1,000Hz, and you simply filter for the 1,000 Hz, now having your entire body moving slightly with 1,000Hz sin.
WHen your finger is on the very narrow end, most of the 60Hz Efield moves to the underlying metal plate.
As your finger slides to the wider part of the tapering metal strip, more and more of the 60Hz displacement current couples onto the strip instead of moving a millimeter further to the underlying metal plate.
And at the very widest part of the tapering metal strip, essentially ALL of the 60Hz Efield from your finger has coupled onoh the tapering sensor strip.
As you press your finger harder, you will get more displacement current, so you need to anticipate that effect.
You need two layers of insulation, and a metallic base plate, and the tapered metal strip which is your sensor.
How much current? Assume 1cm by 1cm area, and 1mm distance form your finger to the taperer sensor. Assume the Relative Dielectric Constant of the insulation is 5. Why not?
C = E0 * Er * Area/ Disrance
C = 9e-12 farad/meter * 5 * (1cm * 1cm) * 1mm
C = 45e-12 * 0.0001/0.001 = 45e-12 * 0.1 = 45.e-12 = 4.5 picoFarad
At 160 volts at 60Hz, the SlewRate is 160 * 377 ~~~ 60,000 volts/second
assuming there are no SPIKES from motors or fluorescent lights.
The current (Maxwell's Displacement Current) is
using Q = C * V, differentiate with dC/dV being constant,
get dQ/dT = C * dV/dT
whihci becomes I = C * dV/dT
And substrituting, I = 4.5pF * 60,000 volts resone sinusoid
I = 4.5e-12 * 6e+4 = 27e-8 = 270 nanoAmperes (with maybe 50% accuracy on this math)
NOte 1 MegOhm shunt, sensed by UNity Gain BUffer, produces 0.27 volts.
For 1,000 Hz at 10 volts, (charging your body thru 1Kohm resistor! why not), yo uget approximatey the same 0.27 volts.