With a little time and patience you can remove the silver backing from your favourite calculator's screen and just replace it with a backlight screen. Just ask at a specialty electronics store.
There can be difficulties with this, though. The rate of battery drain is high, and we all know how batteries like to fail at crucial times.
It can be very difficult to read the display in certain conditions.
Most ready-made backlit devices will be quite large, expensive and probably have functions you don't need.
But then is it really too hard to switch on the cabin light??
Cheers,
Red