Do not use conventional denatured alcohol if it becomes cloudy when you add it to water. This denaturing ingredient will interfere with the emulsion making process. Try to use pure ethyl alcohol if possible with no denaturant.
Thymol is probably the best because it is not very toxic, easy to work with and readily available. It will change the emulsion slightly though as any organic chemical will so you may have to tinker with the forumla a tiny bit. It may not have any effect if you stay below a given level.