What are suitable chemical compounds to make home-made "photoblock"to prevent photo speed cameras/Lic.Plates?

Please do not advise me to go out and commercially purchase this product ( I am askig the "expanded wisdom" of those of my knowledgable brothers,and sisters out here in cyberspace)---the idea is to made "home made" thereby saving money !!! ----Current commercial product @ $19.95 a can ,plus S+H ,seems expensive for a small areosol can which claims will only cover six licence plates.