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I don't think you'll find something commercially made that is small enough to mount on the "blank" cover for the switches. Most often, manufacturers package such devices in bigger casings (to impress customers?), even if the case is 99% empty!
But if all you want are a set of color-coded LEDs to indicate a voltage scale, you can custom-build one easily, and then mount the LEDs through holes drilled in the cover panel. All you need are some zener diodes, resistors, and several different-colored LEDs (3mm LEDs are more compact than 5mm), and the circuit could fit on a small perf board.
The key: zener diodes will block current (to otherwise light the LEDs) until voltage across them rises to a certain threshhold (specific for the zener you're using, e.g. 13 volts), and then the current is allowed to go through; if the current is flowing (and the LED is lit), you know that the voltage is at least that much (e.g., 13 volts). You can get an array of zeners for 12, 13 and 14 volts (maybe even 1/2-volt increments, but I don't know for sure).
The various resistors are just chosen and added to protect the LEDs from too much current.
Have fun.
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