I have three plastic Keter boxes but I'd not store sucions in them as they all let in some rain . Thet are quite useful for plastic ware, and decorating supplies if |I cover such things with plastic basins. I think cushions would get musty.
I keep cushions in my open fronted gazebo from one year to the next, thoughout the winter. I had split bampoo screening put round the sides, and the rare showers that actually get inside the gazebo don't matter much as the place is so airy that the cushions dry very readily and have never become musty. The colours fade, but I rather like faded chintz.