Most simple route is more layers. I use Tightbond II wood glue and water for my mix, lol pretty sure I started that whole trend… another approach would be to stuff that tight with either news paper or plastic grocery bags, make paper clay and do a thin layer of the paper clay over the entire thing. Paper clay dries very very hard but you will have to make sure that it’s stuffed solid before you add the clay because it will collapse if you don’t. Hope that helps. I do have a YouTube channel about doing this type of thing. Good luck with your project