The true polymorph spell says:
You can turn an object into any kind of creature, as long as the creature’s size is no larger than the object’s size and the creature’s challenge rating is 9 or lower. The creature is friendly to you and your companions. It acts on each of your turns. You decide what action it takes and how it moves. The DM has its statistics and resolves its actions and movement.
Looks to me it says CR 9 or lower, and so creating a CR 9 human champion is perfectly legit.
Let the messing up of giants begin.