I think some long held information from the forum got lost in this thread.
Eddy currents form across unbroken connects, like sheet or billet stock. The reason iron filings by themselves is bad is because one flake is in direct contact with others around it, creating electrical paths. The whole point of imbedding it in epoxy was to isolate contact paths. Magnetic fields induce electrical currents, and vice versa. Isolation of the material is absolutely critical, to minimize current generation through the material. Simply pouring epoxy over powder would not isolate it, you have to stir it in thoroughly. You are not changing the direction of flux using the powder, but rather lowering resistance of the path. That flux potential is dependent on the strength and direction of the field through the medium. This is akin to using better conductors to move electrical fields. You may ask why metal to hold the magnets. Because the magnets are static position to the metal holding them, there is no field generated. Someone suggested a second set of magnets, to increase flux strength. Again, thats true as long as the magnets all move in unison. Move the magnetic disks at different speeds (or direction) and you create eddy currents opposing each other. You already know, when current forms in your coils, you have magnetic fields generated. Why are those fields not creating resistance? Actually they do, which is why you never match a coil count evenly with magnet count. The use of neodymium magnets makes this exercise with iron filings and epoxy an exercise in diminishing returns. The most important lesson from this forum is that increasing magnet count is a better use of resources than increases in magnetic strength.
There is a simple test if your filings are not isolated. Heat.