Nun, I recently picked up a used small frame bag, not the ones that fill up the entire triangle, but the narrow ones that are maybe 4inches high, so still allow smaller water bottles in the cages. I haven't really used it yet, but can see how it would be useful for putting my pump, repair stuff and heavier odds and sods in, to free up a bit of space and weight from the rear seatbag I have.
Like all bikepacking stuff nowadays, there is a wide range of qualities and prices for these sort of bags, but there certainly are good quality, reasonably priced ones out there that there.