This is the bare PCB you need to build your own 20mm FET driver!  DIY parts kit available here.

This updated version makes spring bypasses a breeze---simply run your LED+ wire directly to the spring.

Parts Required:

• FET: LFPAK56 N-Channel Mosfet
• D1:  SOD-323 Schottky diode
• C1:  10uF MLC capacitor, 0805 (X7R preferred)
  • Populate CZNR instead of C1 if used zener modified
• R1:  19.1K, 1% resistor, 0805
  • Use 36K 1% 0805 resistor for 2S or 3S zener mod LVP
• R2:  4.7K, 1% resistor, 0805
• UI:  ATTINY13A MCU, SSH or SSU variant
• Z: SOD-323 4.3v or 5.1v zener diode
  • If zener diode is used:
    • populate CZNR instead of C1 with 10uF capacitor
    • replace D1 with 200 ohm resistor
• Spring / LED leads
• OTC:  1uF MLC capacitor, 0805 (X7R preferred)
  • Pad with side closest to MCU can also be used as electronic switch pad (no OTC installed).

The parts layout is the same for the 17mm and 20mm boards.