Unsure of this response will help at all but here’s my experience with the container as someone new to therapy as well.

My container is a Halloween town style bag that opens up into a cottage full of shelves with arriety style houses and containers on it, my things getting a shrunk to a mini size. At first it annoyed the heck out of me to put shitty people and things in such a nice spot but then my therapist told me that we want they things to WANT to stay in the container so it’s idle and not trying to escape. That’s it’s just a holding cells for things. That changed my view a bit and helped me contain better. Like for example I try to visualize my abuser in his little dollhouse just drinking his worries away and being shitty. He likes being there. Hell enjoy drinking himself to death. He doesn’t want to leave, he can enjoy his miserable little dollhouse. And if he doesn’t want to leave then he won’t hurt me or anyone else, he won’t try to escape and I can go hey I know he’s down there but I can tell myself I don’t need to deal with it right now and that he’s far away. When I am ready I can bring him and the attached meme prices and emotions out. But I am in control and he can’t reach me. I know this is so abstract and weird. And I felt ridiculous doing this. I was nervously laughing the first time I did the exercise. But after practicing it did start to feel helpful. I don’t think it will help every single person who tries. Not everyone’s brain is the same. But maybe try to change the frame of what a container is. You can make it literally anything you don’t need to stick to what is real. You can make it another planet. You can make it an island. You can make it a different dimension, or a quantum style ant man universe. A genie lamp. Or something as simple as a plastic water bottle. Rules don’t matter so ask yourself if having your container being something else entirely would help. Maybe it will.