Is it possible to disable the default container? I find myself opening things in it by accident out of habit.
Probably not - the default container is core to Firefox. But maybe one of the issues in the GitHub repo would help your workflow? Add 
and/or comments to any issues you like …
I think the default container would not necessarily have to be removed. I would be happy if C-t and the + button would ask me for a category.
Although at that point the default container would probably be unused. 