One possible way to do this is to open the list of people who have opened and give them a temporary tag. Then you could create a temporary segment of people who don't have that tag. This will give you the list of people who didn't open.
One thing to bare in mind - open rates aren't entirely accurate. There will be some who have open rate tracking blocked. And there will be others who use Apple mail, which will now open all emails in advance whether the reader opens them or not.
