I noticed some fleeces have a "right" side and a "wrong" side, and if you put the "wrong" side up, it wicks, where the "right" side won't. This is especially true for the walmart fleece blankets. Hopefully, that helps!
I had the same problem. I taped the coroplast together, with packaging tape, and then covered the entire thing with a vinyl table cloth to prevent spills from the joining area.