I've got working quite well. The lid of the jar is tied to GND and I attached a piece of foil on the end of a wire to the sense pin.
The main difference was that I ended up running the Tiny85 at 8MHz instead of the 1MHz I was originally using.
1MHz works fine when you're connected to a USB cable but it simply wasn't fast enough to get a good reading with a piece of foil as "ground".