Sound opening for operator panel

I'm the owner/manager of an upstart in the marine electronics business. We have a product in the market where the operator panel (machined aluminum) does not have the desired sound pressure. The speaker within the enclosure has very high sound pressure, but we, nor the contract manufacturer, have not been able to get enough of this sound out. The problem is the different water-proof materials that we've tried, that sits behind the perforations in the aluminum panel, absorb too much sound. Testing without any membrane yields a very loud sound level. The membrane will be exposed to salt-water, sunlight, water freezing atop the panel. It is not a high volume product, so it's not necessarily possible to go to a supplier and get custom solutions with MOQs of 50000 etc. The project as such has a highly variable duration, since it may almost resolve to a sourcing issue, or it could involve more if necessary. We're looking for the fastest route to a robust solution. Very specific experience with similar solutions is critical. Doing a good job on this there's more little tasks to come in the near term, and later there will be larger projects within mechanics - mostly related to encapsulation and operator panels