TK-Fett - Yeah, that's pretty much how I did mine too.
Mirax- I popped off the Rubies rangefinder top after scoring the glued seams
with an exacto knife. I then cut out the two plastic anchor posts inside to make
room for the LED components. I then used the exacto knife to drill out the two
raised bumps found on the top of the rangefinder by digging the tip of the knife
into their center and just spinning the knife (this drills the plastic out nice & clean).
I also cut out a notch in the bottom of the rangefinder close to the stalk for a switch.
I then got two 3 volt LEDs (the blue one doesn't blink, the green one blinks), one
3 volt Lithium watch battery, and a .99 cent digital watch, and a sub-mini slide
switch (Cat. # 275-406A), and some thin gauge electrical wire, all from Radio Shack.
I took out the battery housing from the watch, put in the battery, then saudered the
wire connections to the slide switch. I then screwed the slide switch into the slot by
using the provided screws and drilling in small screw holes into the plastic using the
exacto drilling method mentioned above. I then saudered wires from the switch
to the LEDs, bent them to fit inside the rangefinder, then closed the rangefinder up
(gluing at just certain spots on the inside lid so it would be relatively easy to open
up again when the battery needs to be replaced). The juice from one 3V lithium
battery is enough to run both LEDs for a long time.
Hope this helps.