There are surface mount spring contacts which might do the trick... but end in basically a point. Digikey
carries some.
For prototyping, how about sticking some squares of copper tape to the inside of the plexiglass? If going to volume production, it may be practical to get a conductive film or ink or something applied to the plastics for you.
Alternatively, a different technology approach is to stick a flex PCB to the plexiglass and put the Touch pads on that. Might also allow fun stuff like using a curved panel, or bending around a corner to put 'buttons' on the sides. You can also get quite interesting stuff done on an FPC, like soldering the QTouch chip or LEDs onto it, adding stiffeners for support or simply adding some double sided tape.
Yes, adding an FPC is more parts and money, but it might be a win overall. For example:
- maybe you could shrink your main board if it doesn't need big Touch contacts
- change the main board mounting location and get some internal volume back
- use the FPC to reach some other stuff in the box (e.g. external connectors)
- leave a little wiggle room on the FPC to soak up mechanical tolerances in your design