Hi Amber,
I would suggest to create a Custom Cross-Event Report with the Category: Attendees and Type: Email History. Within the report on the Filtering tab, you can set the Date Field and Date Range to the dates you had in mind. Lastly, you will add an Advanced Filter of Field: Viewed, Operator: equals, Value: No - this will allow the report to filter to only those attendees who have not viewed emails.
Please let me know if you have any questions or would like further clarification.
Best,
Danni