Will my kitten survive the winter?

I have a kitten living in my back garden and over the weeks i have gained her trust and she now allows me to stoke and play with her. I feed her 2 pouches of purina felix kitten food and mix a bit of dry food in with it and give her fresh water everyday. She occasionally comes into my house but does'nt really like it which is why im worried she wont survive the winter outside, i have made her a waterproof box with a inch of poly-stirene going around it so it keeps more warmth in and i have put some blackets inside, will this be enough to keep her warm? Her mother has nothing to do with her and shes only around 12 weeks old, im really worried that she wont make it. If anyone can give me some ideas on how to make sure shes ok it would be much appreciated! Thank you!