Super Modding?

I’ve been playing the game for a while now, and I’ve recently discovered that I start the same way every time. I build a fence around the Evac Shelter, and create a superhero.

I was wondering if I could modify the game, so that all Evac Shelters start with fences around them. I’m also curious about creating new items, mutations, and CBMs. Can someone give me the basics on how to do those?

Creating new items is trivial. Creating new mutations and bionics is hard.

At the moment I think you’d need to modify the shelter function in mapgen.cpp to give it a fence, I don’t think there’s a way to override that one in the json.