You need some way to keep the passengers from being roasted alive, and you need to engineer it to commercial air safety standards (redundant systems, emergency

You need some way to keep the passengers from being roasted alive, and you need to engineer it to commercial air safety standards (redundant systems, emergency exits, little sign that says 'no smoking'). By the end of the process you'll be pining for the days of having to just lug around a little oxygen.