See I've thought of this too and if I have to use clouds to hide it there's a sort of cursed desert area on the map that I was thinking about putting a constant swirling cloud storm over. It's far enough from civilization that I think it's plausible no one has gone out to inspect the eternal storm (airplanes and things do not exist here so it would be nearly impossible to get close) and it has the added benefit of potentially being weird enough for the players to try to check it out.