Dismantle 3 Gametraks and remove the pair of joysticks in each.
Ream out the paddle hole to match the diameter of conductive chord you selected for the strings. I used standard chandelier wire which required a slight enlargement of the hole.
Install the joysticks on the bridge, according to the height and string spacing you have chosen.
Drill the holes in the bridge to pass the chord through.
Prepare your wire by cutting it to length and passing through the paddles and bridge. Create a loop on each end and lock into place by soldering it.
Use screw or dowels to create pegs at the headstock.
Install angle irons at the other end to attache rubber bands to.
Cut your our velostat (or conductive fabric) to size and attach it by glue (permenant) or staple gun on the underside.
Tape a length of conductive tape across each end and solder wires to each one long enough to bring to VCC and Ground connections on the microcontroller.
Install your favorite microcontroller and multiplexers. You have to find a way to acquire 6x3=18 parameters. I used uOSC in a BitWhacker.
(An Arduino would work but the position sensing benefits from the extra resolution (12-bits instead of 10) of the Bit Whacker I chose)
Solder a wire of the smallest reasonable size you can find from the middle of each "short string" to the multiplexer. Leave some slack to accommodate large displacements of the strings.