if you are going down the diy route there is another alternative.
Years ago in the days of the KN3000 I built the 6 switch inputs available in a 9 inch long aluminium box on an aluminuim plate which sat under the keyboard, so that all the large mushroom push switches were under the palm of your left hand. While still holding down chords you could tap a switch with the heel of the left hand quite easily.
I always meant to mount a potentiometer in the end of the box, wire it up to the volume pedal, and have a right angled loop of stiff wire attached to the spindle, a similar arrangement to the knee controls that used to be on old organs, but in such a position you could again control it with the palm of the hand, but never got around to actually building this mod.