Use a mono jack for J1 instead of stereo (if you want), leave out the battery, DC jack, LEDs (D1-3) and resistors (R1 & R2). Still connect the black wire from J3 to SW1, but leave out the orange, red, and brown wires (and also the purple wire to D1). There should be nothing connected to the middle pole (middle vertical column lugs) on each switch, they're only for the LEDs, so you could use DPDT switches instead of the 3PDT's on the diagram.
If I was to make this I think I'd leave the DC power plug off. Because my 9 volt supply is + earth, I could make up a extension lead to change it to - earth but there is always the chance that it gets mixed up. 9 volts won't hurt (maybe) but still a hassle and a 9 volt battery will last a while I think.
Made one up some time ago, with out the lights and I think it would be better with the indicators.