I have, for that reason alone, a Fishman Neo-D Humbucker I got for $80 from Sweetwater. It sits in my equipment bag as a last resort if I have a catastrophic p/u failure. It sounds OK, very electricy but the B string is hot. There is no pole piece to adjust, so it is what it is, and it exists solely as a get-this-gig-over-and-get-me-out-of-here device.

But it meets your criteria of being small, self contained and easy to install with an attached 10 ft cord to a 1/4" plug.