This might be overkill, but...
In the broadcast industry, to shield equipment from RF (radio frequencies) sources such as an on-site transmitter, we sometimes used Aquadag, a thin liquid with graphite in it.
I know it's available in large drums, but here's a source for smaller quantities:
http://www.technologysupplies.co.uk/acatal...int_342144.html
I've never thought to try it in a guitar control cavity, so I don't know how it would work. Copper foil works good:
Good luck!