🚀 go-pugleaf

 You can get piezo tweeters, which are about as non-magnetic as you can get -
aside from the fields generated by the drive current. For the woofers you
are in a bit of a jam - most get their punch from a big cone and a big magnet.
However, since the bass component of sound is pretty non-directional (the
wavelength being long) you might get satisfactory results by mounting the
piezo tweeters in your planned location and the woofer(s) in a remote spot.
I have a 6" polysomethingorother woofer I got at the Shack a few years ago
which looks like it would do OK if the backside were protected (w.r.t. the
salt spray problem). Maybe you can find a location that will not interfere with the compass.

 Another possibility: to try and contain the flux from the speaker magnet
somehow. You might be able to cob up a flux return path as part of the 
enclosure that would suck up the stray flux and feed it back to the poles.