I'm not sure that "The Great Divorce" is intended to tell us Lewis's actual opinions about hell and how one gets and/or stays there. Isn't he at pains, in his interchange with George MacDonald at the end, to insist that it's mere speculation and not intended to be any kind of statement of doctrine?

(It's years since I read it, so I may well be wrong; in particular, I'm not more than 80% confident that what he says there can't be interpreted as "I expect things actually are somewhat like this, but it's important for the reader to understand that I could well be wrong about that".)