Eh? A (aerosol) can of (acrylic) conformal coat is around $15. While it'd be better to dip coat rather than spray coat, it'd still work. And it'd last more than one large-area PCB.
You can play with geometries all you'd like (slanted PCBs with bulky connections up top, allowing them to drain, some kind of high surface area fabric to wick up moisture once it piles up, etc.) but, if it's as you say, you'll always get condensation and moisture on every surface (including your boards). And you won't be able to (passively) drain this fast enough to keep your boards from getting wet. Nothing short of hermetically sealing your box will prevent that (and that's the only 'geometrical' solution that exists).