Adding volume antenna to harrison's minimum theremin

Hey, I'm fairly new to electronics and theremins in general, so I have a few questions about a project I'm trying to take on. I successfully built Arthur Harrison's minimum theremin (the one using hex inverters). I am now trying to find a way to add a volume antenna to the circuit. Can anybody please give me some kind of direction to take. I would like to keep the design similar if possible. Any help would be appreciated.

I have been googling for an answer to this.
Could I use the same type of design for the volume, using new inverters and then somehow convert that frequency to a straight voltage then pass the pitch frequency through an amplifier using the voltage from the volume?

Lacking a volume control you have a semi-theremin! I think constructing an ideal after market volume control add-on would cost more than just a pitch only circuit board. This may be why no one I know has made a public design much less a kit.

You can improvise easily with a finger shadow volume control by using a CdS optical sensor removed from one of those bedroom night lights that plug into the wall. Just put the hacked sensor in series with the output of your semi-theremin and the amplifier input. This will respond very smooth and effectively. Now you have a theremin, well maybe?
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