In order to see whether some of the resonances in the output filter cavities are due to a mechanical self resonance in the long axis, here is a simple calculation to see whether it's possible:

\[\omega = \sqrt{\frac{k}{m}} = \sqrt{\frac{EA/L}{\rho AL}} = \frac{1}{L} \sqrt{\frac{E}{\rho}} \]

\[f = \frac{\omega}{2\pi} = \frac{1}{2\pi L} \sqrt{\frac{E}{\rho}} = \frac{1}{2\pi \ 0.5 m} \sqrt{\frac{69 GPa}{2700 kg/m^3}} = 1600 Hz\]

Here, \omega is the angular frequency, k is the spring constant, m is the mass, E is Young's modulus, A is the cross sectional area, L is the length, \rho is the density, and f is the frequency.

It appears that the worst resonances in the cavity spectra are closer to 4 kHz, so either this is too simple a model or that resonance is due to something else.