This is an interesting assignment, because, as you know, Boo does not speak. He is a recluse, not by choice.
As for the scene, there are many you can choose, but how about when his brother Nathan cements the tree knot so that he cannot put in any more presents? This happens in chapter seven. Up to this point, he is trying to communicate with Jem and Scout in the best way that he can by leaving them presents. So, when Nathan cements the knot, Boo must have been heartbroken.
In light of this, it would be great to have a story about what Boo is feeling. He can express his frustration, anger, longing, and sadness. We can imagine that he is conflicted inside. It can be powerful to hear from a silent character.
Let me start you off with a few prompts:
"Why, oh brother, would you cement the knot..."
Here is another:
"I wonder what Jem and Scout think of my presents..."
"Should I make contact..."