MPX.org has a great audit workbook. It spells out exactly what is required and it references the specific cites in the NACHA book. There is little need for your own workpapers because the audit workbook comes in a binder and each page has the required testwork for say testing on NOC's, so you write your results in the workbook and then I just three hole punch the supporting documentation and insert it behind each page. It is a good source, you might try it. Good luck.