Making a bag insert for tools

How can I make a thick sturdy insert for my tool bag that will keep it's shape while keeping my tools separated?

What thickness of leather is sturdy enough? What tools do I need to see it by hand, thread type to handle abuse? What basic tools. Simplicity is the key for this project.

I'm willing to buy whatever I need and go through as much trial and error as necessary. It's confusing getting started.