I'm completely useless when it comes to this stuff, so this is just me regurging bits that I've heard him mumble-planning (lol) but he was talking about cutting in a window or two for window unit A/C's. Not sure what you use for that kind of cutting (steel, isn't it?) I know there are people who have converted semi trailers / shipping containers into bunkers, and even some who have converted them into fully functional (and surprisingly nice) houses.

Sorry I can't be of more help, but I think it's worth investigating for sure