polymorph:
But you don't -want- more capacitance between the sensor and the lid. ...
As I said I couldn't even say if he needs a cap the size of a dime or a half dollar. It'll depend on environment, dielectric material, the detector. It's an exercise for the student. I'm just offering options and describing how I made them from scratch. Personally I think I'd rather use the piezo/tap because it's probably more uniform over environment variations (carpet, clothing, humidity.)
A high school kid could win a prize with this whole homemade cap idea. Describe and explain the equation, diagram how the electrons toddle about, chart about different dilectrics, make a bunch of little caps that demonstrate the theory, and make a capacitance meter to test and measure them.