My STEREO speakers on my off-road POPUP were sealed in place using sealant... These speakers are mounted from the inside on the back wall of cabinet. These speakers are marine outdoor quality cones and sound good on the outside of the trailer. Have not busted out any cones during my heavy use since 2008.
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I did install a box cover on the inside of the cabinet wall covering both speakers. This was to help with cold air being present around the open speaker cones... Probably acts like a WOOFER box as well for the speakers...
Roy Ken