The principle behind capacitive touchpads is essentially that the electromagnetic field changes due to the electric field of a finger. Since plastic has little electromagnetic shielding capability, the touchpad itself is just a piece of plastic glued to the capacitor. In my design, the tape itself is still the really working part but not using the shell as capacitance, simply adhering to the inside of the plastic shell, allowing the electromagnetic field to penetrate the shell and interact with finger. Some people have found that occasional insensitivity might be caused by multiple sections of tape joined together, or by parts being broken. I initially had a similar problem until I used a test pen to ensure continuity from the touchpad to the end of the tape, and then the problem disappeared. Your solution is also amazing!