Harrison Minimum Theremin 101 - Moddable to Use Volume Antenna?

I'm looking for a simple theremin kit to experiment with that's not too expensive. The Minimum Theremin from Harrison looks pretty good, since it's cheap and is battery powered. The only problem is that there's no volume antenna. Does anyone know what it might take to add one to it?

http://harrisoninstruments.com/101/101_description.html

A volume antenna (+ circuit) is makeable, but it depends on your electronic skills. In this thread (http://thereminworld.com/forum.asp?cmd=p&T=3119&F=3) I started developing this with a student from Finland. If you want, we may continue here.

It would perhaps be interesting to develop together an independent circuit with volume antenna, audio in and audio out in a separate housing as addition on lots of cheap pitch only theremins.