All thoughts to keep in mind for prospective installations of the Butterfly vario or if ClearNav or LX ever use their sensor suite. Jon must be one of the first to go for it! I only have one flight, highly impressed.
Neodymium (we used to call them Unobtanium) magnets as used in the Tangeband speaker Krasw suggests are excellent. They focus a very high percentage of their magnetic field onto the voice coil, reducing the size of magnet required. Generally the magnets are quite deep compared to ceramic magnets and won't fit in tight places like the factory speaker compartments of Schleicher gliders.
With the current trend in instruments, controlling magnetic interference is going to be a more common problem.
Perhaps look for an 8 Ohm nominal Unobtanium speaker, or wire a 5 or 10W 2-4 Ohm resistor or a capacitor (simple HP filter, perhaps Krasw or Richard could suggest a value) in series. Or just don't turn it up too loud, easier said than done.
Jim