I'm using a red LED hooked up to a 9V battery and a resistor, held together by a custom designed 3D printed part. here's some photos. Since those photos were taken I've added an on/off switch for ease of use. Having a small area of light is good enough, but it's a bit annoying to chase down the exact reflection point - if I were doing this again I'd use multiple LEDs with some plastic or paper or something in front of them to get a larger light source.