Raise or suppress the resonant frequency of the panels such that they wont resonate in the audio band. Per usual, there are tradeoffs. You can stress the panels outward like Acme, pull them in under tension like ?, add panel braces ala fEarful cab style, or just make really thick and heavy panels like most commercial cabs. Whatever accomplishes the job, but as was mentioned some methods weigh more, some are easier to construct, etc.
Acme, bracing and damping - YouTube
