Hi @SeanMcC,
This is a great topic and I think it actually opens it up for alot of discussion about unopened emails and how to increase the open rate.
Out of curiosity, have you done any A/B Testing on your Abandoned Cart emails?
It could be as simple as your subject line or send time for those emails that generate an increase.
Here are some best practices on A/B testing and for reference a few tips on completing the testing.
I too agree with @David To that your suggested strategy of implementing a popup for non-openers is a good move, but this will only account for those that naturally come back to the site to see the popup.
Another suggestion is a exit intent pop up, so this would show prior to leaving the site when someone has not completed a sale. However I would strongly suggest monitoring this and measuring the success to ensure it is providing positive customer experience.
Hope this helps!
Kylie