Bottom line: I need a day bag that can collapse and fit into a larger backpacking backpack.
Context: I tend to travel with a 35L backpack which fits everything nicely. Up till now. I have a very scrappy day shoulder bag that collapses to almost nothing. This allows me to pack it and then transfer a few things into it for day travel. It doesn’t hold things very well, and could easily be improved, but searching around online / in stores hasn’t yielded anything I like.