If you look at how John did his fluxgate, rather than waiting like I
did, looking for a big cylinder magnet, he just glued a bunch of small
rectangular magnets around the inside of a ring shape and voila', he had
a cylinder magnet of whatever shape he wanted.
I am building one now using the rectangular magnets from Radio Shack (5
for $1.79US)...but mine will use 15 magnets...
The point is the flux lines blend and you get the same thing as if you
had a big cylinder that was solid...you could also make a smaller
version with a tighter field for more power...check out John Bedini's
fluxgate construction at;
http://www.keelynet.com/energy/bedgate.htm