I would suggest returning to your introduction. Your conclusion should be a drumbeat that echos in the reader's mind after s/he finishes reading. There are several ways to achieve a powerful ending, but I have found the most effective methods are tying the conlcusion back to the hook from your introduction, or finding what journalists call a "kicker," a quote that you can use at the very end of your essay. If you choose the kicker method, though, be sure to set up the quote at the beginning.

I'm curious if you mentioned the Robert Frost motif in your essay. This could be another powerful way to reference the text and your main idea in the last sentence. Maybe you could tie in the idea of being a traditional hero with "staying gold," and relate their inherent flaws (not being model citizens) with the circumstances they have to rise above in order to become the heroes they are in the book.

Best of luck!