With the (Eureka)Sound Hole Pickup Mount you can add a pickup to your acoustic guitar or acoustic bass quickly and easily, with no guitar modification.
This model is for sound hole sizes of 3.5" (89mm) to 4" (102mm), and for pickups that are "humbucker-sized".
You'll need to bring you're own cable to connect to the pickup.
These work well.
You can see another version that EurekaSound LLC offers that includes all electronics here:
http://www.eurekasound.com/eshpm