Custom Folders
Add folders. I would make a folder called "Box 1" to show which comics are in my first box that hold by physical comics, and "box 2" etc.
Tags are essentially folders.
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Scott commented
This can easily be done now by simply using Tags. Then when you're on the My Comics page you can filter using My Tags to see what is contained in each Box.
As I tend to purchase a lot of issues each month, I store my comics in boxes by Release Month rather than title, so in the Tags field I enter Box: 2019-03 as an example.
I also use Tags for tracking where I purchased an issue: Store: Lookin' for Heroes.
Use commas to separate your tags.
Give it a try it works great and doesn't require any changes to the existing site.