Tall order!
Hmm. Well, ever take a look at some of the boards that are put into radios, they are sealed using a spray epoxy/Sealant similar to Tallmans post. It's already there. Unfortunately many parts being sealed with it can no longer ventilate and can possibly build up extensive pressure an potentially explode if passed beyond their electrical limits.
I have several radios here that both Midland and Cobra - using Vertical Board design - that used this spray-on epoxy or shellac equivalent. Applied directly on the boards themselves after insertion and thru the soldering process. It's for the purpose of adhesion, sealing and curing - when they are jolted or put under stress beyond a set limit - these boards withstand the torque, but break the seal - and even lift off the traces of the board from it's edge on connector.
IF you look thru older marine radios - you may see how they designed a grommet and spacer washer system used on shafts and outer surface load bearing places - where dirt and water can collect, they "Gasket off" A simple rubber washer can help - just place or seal the washer to the non-moving side and let the knob or switch rotate past - as long as the part can maintain some contact with the gasket - you're pretty safe.
PC-122 as well as TRC radios used a similar "foam sponge" in switch knockouts to keep dirt out and control moisture. You can't make it air-tight, but anything you can do to help keep water from getting at most of the electronics can help - it's why we have that silly wax museum in the VCO sections of the typical CB.
Also you'll need to address how to keep water that GETS in there, TO DRAIN back out of there and continue on its' way.
That means drip loops in wire looms as well as removing stuff that is Hydrophilic, (water affinity) like paper gaskets, cones and stickers and or slip cover wedges and shims to keep down rattles. Yes, even Speakers, they have to be swapped out for polycarbonate or plastic cone types to keep moisture or water from direct entry into the cavity.
It's never easy, and why these types of moisture / Shower proof radios and such are so expensive - unless it can be sealed completely with one big giant rubber gasket or ball - you are at the mercy of the elements that can work their way into your stuff you're trying to keep from getting it.
