You could use a clip-on miniature microphone ($27 at Radio Shack). It has to have a line amplifier, as the RS one does. A plain unamplified electret mic puts out too weak a signal.

If you want to attach it outside you'll probably want to rig up something that's easier on the wood than the (removable) alligator-jaw clip it comes with.

Another option is to mount it inside. That's what I do. You could get a soundhole cover (or make one -- I've never seen one for sale that fits classical guitar soundholes) and then make a hole in it for the microphone's tiny cable to poke through. Mine is mounted permanently inside, but that required drilling a hole. I think it sounds great, with no feedback and very natural tone. You need the soundhole cover if the mic is inside, not just for mounting it, and not just to limit feedback. Having it inside without a cover makes the bass boom.