I have actually found St. Thomas' work on natural law to be quite sane, sensible, and corrective in many areas:
Eternal Law: Includes God's foreknowledge and creation of human events/contingencies. Largely inscrutable to man.
Natural Law: appropriation of rational creatures of the divine exemplar; inner apprehension of right.
Human Law: Constructed from natural law and conditioned/applied in historical circumstances. I have always wondered if this could parallel general equity.
New Law/Old Law: Figure it out on your own.
Interestingly, Thomas does allow in some degree the OT model to be a model for nations:
On Kingship, 105, 1.