The main thing is the connection between the master-volume-pot and ground (marked with „1“). The higher the resistance, the higher the volume. If you cut the connection, you will  have maximum volume.
So I cut the connection , and solded a second pot between the 1M master-volume and the groundbar. Several experiments showed, that another 1M-pot works best (smaller resistors worked well at bedroomlevel, but not in a band-context).
I ordered a 12V-relayboard on ebay (8€) and used it to bypass or activate the second volume-pot. You can use the contact “U” (marked with “2”) for power-supply.
The already-assembled relaycard has a LED to indicate the relay’s status. So you can see wether the volume-boost is on or off.
There are several ways to switch the relay itself (powersupply on/off). I decided to use the footswitch plug that my OTS Mini already had (marked with “3”).
Contacts Y1 and Y2 switch channel and OD-boost if connected to ground. Z1 and Z2 are just power-supply for the LEDs.
So I disconnected Z2 and used it for switching the relay. Z1 now supplies all LEDs in the footswitch.
I still had a 3x footswitch (from a TC G-Sharp), but you can add a third switch into you existing Ceriatone-footswitch as well.

A perfect place for the volume-boost-pot was the existing hole in the chassis, where the power-indicator of the amp was before (see photo above). I placed the relay-LED directly above it.
The power-indicator moved above the power/stand by-switches.