The passive optical volume control can be placed anywhere, even by your foot.

The image shows my theremin as it is when stowed in it's combination legs/case. The optical volume is disassembled to show how it goes together. Normally the optical volume remains assembled and nests beside the theremin when stored. The knob beside the optical volume control is a carriage bolt through a wood disc made with a hole saw and is used to hold the leg on. There are T-nuts in the bottom of the theremin that the knobs screw into. The top of the volume control can accept a piece of pipe or any 2" PVC fitting to modify the light path. 
Or a cap to act as a mute.