Soft top boxes?
 

About 10 years ago my Dad found an almost unused "Endurance" brand tail pack that's like a soft top box - big, square, keeps it's shape on it's own - in a charity shop for a whole 3.50. It has been absolutely fantastic (and it's still going well), so it'd be great to find another one for my other half, and maybe a replacement for when it finally goes too, but I've never seen another bag like it.

Does anyone have any leads on other manufacturers of this sort of thing?

Givi has options in various sizes: WL901, XL08, EA147

just tie a large expandable tank bag on the luggage rack, much more versatile

If you search for tail packs or tail bags you may find something that suits.

I'm going to have to take a photo later, and explain this better. It's entirely soft (no breakable hard base), but holds it's shape so you can just chuck stuff in (very handy when I've got a bag of groceries to take home). It keeps it's shape due to having a layer of foam inside, which means that if I put a bottle of pop in it straight out of the fridge it stays cool for the day even when it's 35º or more.

It's bigger than a tail pack, it can just about take a 6 pack of 1.5L bottles. The outside is hard canvass that has survived sliding down the road a few times and living under the Spanish sun for years, it's water resistant, it attaches directly to the bike and doesn't need any extra straps or bungees.

I have normal tail packs, and if I go away I typically use a sports duffel bag, but those don't fill the role of a top box while also being unbreakable if it lands on a rock.