Don't use the white foam made up of the little balls of foam together, those things crumble up.
I also top off the straw with an old mil surplus wool cot blanket
I use a clear silicon caulk around the inserted flower pot to hold it in place/weather seal it
www.youtube.com/watch?v=YpG3...
youtube.com
DIY Outdoor Cat Shelter
YouTube video by ARL BerksCo