Hi Boy Nigel,
The Boss FS-5U momentary switch will work as a single AUX switch for the H9. Make sure to get the momentary switch, not latching, and set it to "normally open" with it's polarity switch:
For a smaller form factor, T1M makes a single momentary switch:
This one from T1M hasn't been tested by Eventide, but in theory, it should work. Make sure to order the "normally open" version if going this route.
I hope this helps!