I've used magnets in al my 15mm tanks (both FoW and Sci-Fi)
Even on these (and they're really small) :

I drill a hole the size of the magnet in the pivoting point of the turret and of the tank. Too much off centre will give you an akward rotating turret !!!
make sure the magnet sits level with the surface.
Check the polarity before you glue it in place! you wouldn't want your turret to be rejected.
I then glue the magnet in place with TEC7. (very good stuff)
And that's it.
Neodymium magnets are very strong, so to economise, you could try using just 1 magnet and a piece of (ferro)metal.
P.S. I got my magnets from
http://www.supermagnete.de They got all sizes and shapes.