Many people are fond of Michael Smith`s formula, which is basically a modified amidol/citric acid formulation. Based on the information I have read and the loyal folowing he has I suspect his would be the best formula for AZO.

OTOH if you feel like experimenting I have the theory that pyrogallol can work just as well and much cheaper than amidol. When pyro is used as a non staining developer for paper it has the same charachteristics as amidol and of course the price is much lower.