Old question, but a new option to investigate.
UDO has a new product just out, a ribbon they say can be plugged into any expression pedal input, and be used with that synth.
This would not work with the Explorer ( no expression pedal input ), and there would not be a Theremin mode, and it’s TBD how specifically it works with the HSD. You could use this with other synths ( the Super 6 and Super 8 ) as well, even as a second ribbon on the larger keyboard Hydrasynths.
It has magnets on the bottom so it will stick to a ferrous case.
Length 581 mm, 23 inches.
Scheduled to ship in December.