Has anyone used these pen trays? Are they relatively sturdy or are they really thin/flimsy? Can they be stacked? If anyone has these, I'd appreciate exact external dimensions. Thanks. http://ecx.images-amazon.com/images/I/31qRpaVE%2BZL.jpghttp://ecx.images-amazon.com/images/I/41Xmt4XJEeL.jpg