[Scirion]

Or, you could just do it this way:
Stolen directly from airsoftarizona.com.

Sick of buying expensive batteries for your red dot?

Do like I did and make it run on 2 AA batteries, heh

I drilled a hole in the battery cover and made a false battery out of a big chunk of pencil eraser and 2 thumbtacks. I then soldered wires to the thumbtacks and ran them out the battery cover. Last was to stick a battery pack on the side of the gun.

The wiring is a little sloppy, but it gives me some slack to eventually stick the batteries inside the forearm. (2 AAA batteries would work too, but I couldn't find a sealed box for those)

I shouldn't need to change the batteries for, oh.. a year