Write a monologue for Atticus after his closing speech in the trial, about 3-5 mins
Rather than just writing this for you, let's try and give you a bit of help.
First of all, what's the audience at this point? He's just given his last argument, and is waiting for a verdict to be returned from the jury. Who would he be talking to? Perhaps the sherriff? Maybe his children? Tom Robinson, his client? Surely he'd speak to Calpurnia to check in on the kids...
There's a lot you can do just by thinking of the audience. Next, look at the speaker. What kind of person is Atticus? As a respectable person, what is he likely NOT to speak about, as opposed to what others in his situation might say? He probably wouldn't discuss too much of the trial specifics with anyone other than Tom, but he might have something morally important to say to others, especially his children.
Finally, what would the purpose be for his discourse? By thinking of this, you'll end up with a great situation for writing. Whether he's talking to Calpurnia about how his kids are holding up, or to Heck Tate about the gratitude he feels for his honest testimony, you've got plenty to write about once you consider these three aspects: speaker, audience, and purpose.