How would one insulate a makeshift shelter during winter? Figured this is the place to ask.

Im planning a trip to the woods my grandma owns when i turn 18 where im planning on building a makeshift shelter and spending a night there. I've learned most of the basics of shelter building and lightning a fire from my dad, but im still not super confident.

In the summer/fall it would make sense to use dead leaves, and although using evergreen branches is an option i don't really feel like cutting up trees if i can help it.

The plan is to use this well type thing (basically just a hole in the ground with concrete all around) that my dad used to play around as a kid as a fire pit to cook food with, and have a lean-to/tree well type shelter going on (pic related) with of course some other obstacles to block the wind from coming from open holes on each side

Dry leaves are out of the question because of the snow covering the ground, and as i said before im hesitant on slaughtering trees just for the branches.

I'm planning on bringing a knife, the food and a lighter, and if i need an axe, a saw or any other tool i can borrow it from grandmas place, so i have basically all the tools i need