If you're at this website and you're a PMP, then no doubt you have access to the free downloads on PMI.ORG for the various Books Of Knowledge. I would go there first and download the book "Business Analysis for Practitioners - A Practice Guide". Just like the PMBOK, it is a very structured book and I believe that this would be your best bet to build your Excel template to document your BA experience.
Like most templates for this purpose a simple formula, which can be put into Excel columns should suffice. Basically a column for "Opportunity", "Expected Costs if the Opportunity was not addressed", "The tools/methods used to address the Opportunity" and, finally, "The realized benefits and saved costs from addressing the opportunity". Of course you would include columns that include the company/industry as well as the date of when these Opportunities presented themselves to you.
I do hope that this helps.