Not quite what you’re looking for, but the sea to summit ultra sil daypack is a great bit of kit. I have one in my panniers for the unexpected bulky acquisitions. Folds away to the size of an apple and expands to take a couple of kilos of apples.
If you're on a budget you can get similar packable backpacks from decathlon for about a fiver. I always end up picking one up when I go in there and later stashing it in one of my bags.
Not quite what you’re looking for, but the sea to summit ultra sil daypack is a great bit of kit. I have one in my panniers for the unexpected bulky acquisitions. Folds away to the size of an apple and expands to take a couple of kilos of apples.