What material can I lay over a capacitive touch screen that will still make it work?
It just needs to be thin. The capacitors on a capacitive touch screen appear as a series of very thin (so thin you can see through them) metal contacts on the non-touched side of the screen. The change in capacitance developed by your finger in proximity to these is what it senses, so as long as the material is thin enough to not interfere with this, it should be fine. This is the principle by which screen protectors work.

It should be noted that most capacitive touch screens require a minimum finger area (to reduce false positives) which is produced by the action of squishing your finger against the screen.

I would suggest some sort of thin plastic sheet (less than 1mm thick) with an adhesive backing so that it gets really close to the screen, although in my experience the glass is easier to clean.