data driven pages - Creating index layer for multiple maps in ArcGIS Desktop?

I am trying to create an index layer of just the area within the Orange lines. I am having difficulty creating this layer due to it not being a simple box area.

The goal is to create a index layer, with grids, so that the Data Driven Pages would be used to generate it as a mapbook for the response area for my firehouse.

Below is an image of what I am speaking about.

Any ideas?

I am using ArcGIS Desktop.