I've used a few different pickups in my Stella. My favorite was a dogear P90 which was screwed right into the top. Put the ears inside the soundhole and just ran screws through the top. I also had a weird minihumbucker out of an old Japanese guitar that sounded kind of cool, but the pickup was heavy enough to mess with the acoustic tone, so I switched it out for a Seymour Duncan Antiquity strat pickup I had laying around (which is now in the soundhole of my old Gibson 12 string).
Here's how the P90 looked:

And here's the strat pickup in my 12 string:
