I had the same problem with a rubbermaidador. . . . I used a small block of floral foam, with Prop Glycol/water 50/50, and I lined it with cedar. . . . It was always 76-78%. . . . I took the humidifer out, and a month later, it's still at 68%. I think it's the cedar, as I had wiped it down with a damp rag at first. . . . . I think the cedar absorbs the moisture, and it's acting as a humidifier until it dries out. . . . and if the box is sealed, it won't quickly dry out.