What material should I use for the plates of my homemade mica capacitors?
What material should I use for the plates of my homemade mica capacitors?


Gold leaf sounds like a good candidate.
This can be as thin (and so low cost) as you wish - gold leaf can be rolled out and rolled out and rolled  .... - to the extent that (I'm told) that you can see through it. At that stage (I'm told (again :-) )) it's green. 

Gold is nicely malleable and could be rubbed/worked into the surface.
You can also join pieces of gold leaf together by mechanical rubbing.