You want the absolute authority on flat earth, Dr Samuel Rowbotham and his book, Earth not a globe. The Bedford level experiment, true distance of the sun, the true shape of the earth, and position of the earth in the universe would be good starting points.
You could also check out 100 proofs earth is not a globe by William carpenter. Those are pretty short passages that can easily fit into what you are looking to do.
But Dr Rowbotham is definitely a must, his work is revolutionary and the knowledge you will be introduced to will be eye opening.