This is an enclosure for a gauge driven by a cheap (< $5) Switec X27.168 stepper motor. Throw in a printed card and a controller (you can get away without a hardware driver!) and you have a gauge.
I made a Cessna RPM instrument (https://youtu.be/t_wZV1MmZg0) for my simpit, but you can make anything - a thermometer/barometer, steampunk CPU/Memory/Network indicator, many other aviation instruments for simulators, a random number indicator or [fill in the blank].
For controller, I used a $5 Arduino Pro Mini clone (a single board can drive 3-4 motors) which I hot-glued to the back of the gauge.
As for software, if you use Arduino controllers then be sure to check out open-source library https://github.com/clearwater/SwitecX25 and the website of its creator http://clearwater.github.io/gaugette/
For avia simulator applications specifically, assuming that you are in the X-Plane camp, check out ArdSim plugin http://svglobe.com/arduino/ardsim_out.php - it simply rocks!
Place the motor in the motor-shaped cavity. The motor axis will be in the center of the gauge. The motor pins should go through their holes on the back of the enclosure. The motor body should be flush with the face of the gauge.
Glue a printed paper card to the face of the gauge.
Place the printed needle on the motor axis. You might need to print the needle in a different color for better visibility. Alternatively - and that is what I did - you can just paint a part of the needle.
Connect the controller to the pins, flash the software - and enjoy your shiny new gauge.
I just eyeballed the motor and iterated the design a number of times until it worked. The measurements are not exact, but they were good enough for my instrument.