Firstly I changed the mode from 2 to 1 - there is a little switch inside on the circuit board that sets this (bottom of picture).

Then I swapped over the wires on the two control potentiometers on the right hand stick (left in this picture taken from the back). Three wires on each - a red, either a white or a blue, and a black. Note where they go, unsolder, resolder in the same order on the other pot. (You might actually only need to swap the white and blue wires, the reds and blacks may be common supply and ground for both, but for me it was easier to just swap all six than try and trace the wires back through the connector and onto the circuit board to find out where they went.)

This now gives me port and starboard throttles on the left and right vertical sticks, rudder on the right horizontal and bow thruster on left horizontal. On loss of signal the motors and thruster go to neutral (stopped) and the rudder goes hard over (to port I think) but at least the boat will just drift and not go blasting around in a backward circle.

I also have both vertical stick movements on ratchet instead of self-centreing, and have filed a central detent in the ratchet cam so stopped is easy to find (not shown above - this picture is of an unmodded unit).