OK, I don't know if this is a good idea or not, but I had it and wanted to write it down as best I could.
It looks like a small, over the shoulder, leather purse or satchel. Black, because black is cool. It's also rare and expensive. Maybe you can only get it after a grueling quest. Maybe you can only make it yourself after using numerous skills from the skill tree (creating leather by combining meat and paper in a cauldron, then stitching it together with thread, finally pouring a Door Potion into it, using a construction tool at some point in the process -- and it's gotta cost imagination to make it rather that providing a reward at the end of the process).
The bag of bags can hold ten other bags or boxes! You can't open a bag that's held within the bag of bags, of course. You have to pull it out first. And things you pick up or donate won't go to or come from the bag of bags. Putting a bag in the bag of bags is a good way to protect items you want to keep safe from accidental use!
bonus: For added fun, if you drop an empty bag of bags on the street, it activates the "enter" verb. This opens up a rather unique party space that's full of dust bunnies and bits of fibre! (a bag of bags with Glitches in it glows from the opening -- maybe it can be picked up, maybe not -- maybe there's a time limit)