Although many institutions use a "log" format to maintain the required records, there is no requirement that the records be kept in a separate log. The requirement is that the records be made and maintained (for five years) in a manner and format allowing prompt retrieval.
The elements of the record to be maintained differ slightly depending on whether the purchaser of the monetary instrument has a deposit account with the institution. For your credit union, that would mean that the purchaser would need to be more than a member -- he or she would need to have a share draft account or other deposit account with the CU. For this purpose, the purchaser is the individual making the purchase, even if it's on behalf of someone else. For example, suppose that ABC Electronics LLC has a business share-draft account with the CU. Its managing member, Alvin B. Chase, comes to the CU to purchase a $5,000 cashier's check with cash. If Alvin Chase has a deposit account in his name with the CU, you obtain and record information required for a deposit customer. If Alvin does not have a personal deposit account with the CU, you obtain and record the information required for a non-depositor.
The requirements for the record are found in FinCEN's regulations at 31 CFR part 1010, section 1010.415(a)(1) for purchasers who have a deposit account with the institution, and in section 1010.415(a)(2) if the purchaser does not have a deposit account with the institution.
Contemporaneous purchases of the same or different types of monetary instruments totaling $3,000 or more are to be treated as one purchase. Multiple purchases during a business day totaling $3,000 or more are treated as one purchase if an individual employee, director, officer or partner of the CU has knowledge of those multiple purchases.
Records are to be made and retained when the CU issues or sells a bank check or draft, cashier's check (or teller's check), money order or traveler's check and currency in amounts of $3,000 to $10,000 inclusive is used to complete the issuance or sale.
You can read section 1010.415 at https://www.bankersonline.com/regulations/31-1010-415.