ITA Matrix is free and the better search tool for this. You can do very flexible calendar searches for flights on Delta metal in I or Z class.

However, I highly doubt you will find anything. The companion certificates must be flown entirely on Delta metal, and prior to Christmas peak season starting on 21-Dec-24:

• CZM is not served by Delta
• BON and RTB are 1x weekly out of ATL

Also, Delta really, really hates selling connections through ATL unless you are a "spoke captive" in the South dependent on Delta's ATL pricing. They will block inventory and often only sell D fares or higher on ATL connections from elsewhere. Combined with a single weekly flight, I would be surprised to see that you saw even one option.

I did a quick search - the only outbound available in lower than D before the Christmas week is 21-Sep-24 DSM-ATL-RTB.