Hw to make a concrete slab that floats in water it shd have an minimum thickness of 2mm?

i need this for a competition and it must be of 12cm*15cm in area.... so i need this for a robotic event the main point is the base should be in concrete and i vl be using this to hit a ball that floats on the pool to a target at some distance nw i just want to make a concrete slab that floats in water