My previous eigenfrequency analysis was incorrect by two orders of magnitude due to the misuse of Young's Modulus information for Viton. After editing this parameter (as documented on 7/14 19:00), the eigenmodes became much more reasonable. I also discovered the Deformation option under the Surface Plotting Options, which makes the eigenmodes of the single stack much more apparant.
Attached are pictures of the first four eigenmodes:
First Eigenmode: y-translational, 7.49 Hz
Second Eigenmode: x-translational, 7.55 Hz
Third Eigenmode: z-rotational, 8.63 Hz
Fourth Eigenmode: z-translational, 18.26 Hz