Cut two pieces from 3 mm perspex plastic (or similar) or plywood. They should be about 250x40 mm.
Cut pieces from self adhesive conductive metal tape, copper or aluminium. They should be about 18 mm by 50 mm for the main keys and 9 mm by 50 mm for pinky keys and octave pads. Just make a bunch... but at least 6 of each.
Start mounting the larger tape pieces onto the top side sheet. Start 40 – 45 mm from the top and place the LH pieces with 5 mm separation and an offset of 10 mm from the LH finger side. The overshooting ends of the tape pieces are folded over to the back of the sheet. Make a 25 mm space and then do the same for the pieces for the RH keys. See pictures!
Tape the narrow pieces in an angle cutting in a couple of mm in front of the edge of the bottom-most keys of each hand. Use scissors to cut the end in parallel with the keys. Next to the last one, put another one. Again, see pictures!
For connecting electrodes, solder small pieces of copper tape onto the stripped ends (4mm) of regular insulated copper wire for electronics. This is done very easily if you solder onto a longer piece of tape that is cut after the soldering.
Mount the electrode connectors on the electrode pads where they are folded over on the back of the top sheet. Put an extra piece of conductive tape over the connection if you like, for good measure. Route the wires towards the bottom end of the sheet. Use kapton tape or fabric tape to hold them in place.
Cut, strip and solder the wires to their pin holes on the Teensy. Solder on topside to get a flat surface on the back for easy tape mounting to the sheet later. Just solder in the keys from top to bottom to Teensy ports 17, 4, 3, 18, 19, 22, 23 1 and 0. Make it so that the Teensy can be placed 3mm in from the bottom edge of the sheet. Solder in electrode wires for the octave pads too, so they can be easily connected later. Octave up is on 15 and octave down on 16.
The MPX5010GP sensor is a bit bulky, so we use a dremel to cut off the mounting brackets on the sides, and we also make a diameter 14 mm (approx.) hole in the bottom sheet where the more narrow circular part of the sensor can fit. Offset the hole so that the tube connector is centered and protruding at the top end of the sheets.
Connect the sensor the the Teensy. Pin 1 of the sensor (the one with a dent) to pin 21 (A7), pin 2 to GND and pin 3 to 5V.
Use double sided adhesive tape to secure the sensor and the teensy to the top sheet.