The EHX triple aux switch should work. I also recommend the products from Analog Endeavors; make sure you get the boxes wired for Eventide. The easiest way to program aux switch mappings is using H9 Control. Go to the Pedal Settings > General Settings > Use Expression, Aux Switch, or Both and make sure it’s set to “Aux Switch.” Then, go to Pedal Settings > Aux Switch Mappings > and program increment and decrement to the switch you want.
Personally, I go into the System Settings from the front panel and use the H9’s LEARN feature to automatically sense the switch you’re pressing. Page 34 & 35 of the manual explains how to use the LEARN feature.