The Jandd duffel bag would be handy for me. Is there something like that available in the UK? I saw Carradice make one but it's quite a bit longer than I need. I prefer cotton duck over cordura. Maybe I could find something like it in an army surplus?
this:
https://carradice.co.uk/shop/accessories/bagman-saddlebag-supports/?v=7516fd43adaa
with this:
https://carradice.co.uk/shop/accessories/bagman-support-struts/?v=7516fd43adaa
is much better.
i personally use the quick release one with the swift zeigeist bag and it's flawless. Everything you need other than the support struts can be found here
you could probably get away with other pedestrian duffels, but a traditional rucksack with shoulder straps seems unlikely make you happy as it is likely to flop around and have dangly bits everywhere. This one is probably what i would pick if i was trying to get off-bike friendly. because it has a rigid HDPE liner.
.