I wouldn't use a tissue box. There will be tissue fibers left behind that aren't meant to be eaten, there may be a fragrance added, and the glue would concern me. I have used cereal boxes in the past. But a hay feeder of any kind can be a safety issue. They like to stick their faces in the hay, and when it's crunched up in a box, you run the risk of hay pokes to the eyes. I just make a hay pile on top of a wee pad. They can sit there, push the hay around, and moisture soaks into the wee pad keeping the hay drier.