A passive crystal set with good front-end bandpass filter (Chebyshev 10-pole with 3 dB points at 118 and 136 MHz), a sensitive Schottky detector and hi-z phones or 32 ohm buds with audio matching xfmr, the headphone cord serving also as antenna (trim cord to 1/4 wave resonant length mid-band, coupling/decoupling via caps & RF chokes) and you're in business.
Use a metal case for a counterpoise (w/body capacitance coupling); insulate the headphone cord shield from case.
No batteries, no tuning, no on-off switch, no volume control, no interference potential. It's never "on", power supply-wise, so nothing to turn off.
This is the best way to hear all in-band activity without tuning around, missing brief transmissions in the process--think of it as a scanner that gets everything at once. Segregation is by volume; e.g. closer transmissions are louder, so your pilot will be vs. ATC.
For ultimate 'stealth', build it into a tiny, innocuous-looking case like from a gutted mp3 player (if questioned why it doesn't work, say "needs charging"), keep it in your pocket, and use only one earphone--cut the other one off. Keep that one on the side of your body facing away from the aisle for less visibility (window seats rule!) That way you can also still hear flight crew instructions, etc.