But you must remember that the natural law philosophy itself assumes the existence of God. You’re not very likely to find many atheists who accept it.
Natural law is a teleological philosophy; that is, it assumes that we (and the universe) have an objective purpose. Without God, nothing has an objective purpose: everything is accidental. This is why it is impossible to find a logical ground for objective morality (good and evil) apart from God. There is no reason to assume that survival, health, happiness, etc. are preferable to non-existence or total destruction if everything exists for no ultimate reason.
You are right. However, I contend that it is possible, (albeit, difficult,) to prove the existence of the natural law without God.
Whether the exercise has any merit or not, is a good question, but I think it is possible because there are many things in the world which are self-evident. If I touch a hot stove, I feel pain. If I kill my mother, I feel remorse. Yes, the former is a physical truth which can be explained by science and the latter a truth of conscience, which is intangible, but I think that it is definitely possible to prove to any person that there is a natural order in the world, an inherent moral code in man.
The virtues of the natural law are, of course, different from those of the moral law. For the Greeks and Romans, strength, intelligence, stature, and courage were the foremost virtues. Why are these natural virtues? Why did every pagan culture which lacked a separate moral code which superseded these natural virtues consider them to be the fundamental virtues of man? Human nature.
I understand that going through this whole thought process is somewhat fruitless when there is no final spiritual end to look for (God) but nevertheless I think it is useful in proving to an atheist or agnostic that there is still a natural order to the world.