If you have a search around there’s a post somewhere on the internet where someone modified the bag frame – it’s a simple tube design.
Then you can just find a normal casual bag you like, and attach it.
EDIT: here it is, it's for panniers: https://www.instructables.com/id/Brompton-pannier-adapter/
I've just attached a carradice saddle bag to it and it works pretty well.
@hugo7 started
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If you have a search around there’s a post somewhere on the internet where someone modified the bag frame – it’s a simple tube design.
Then you can just find a normal casual bag you like, and attach it.
EDIT: here it is, it's for panniers: https://www.instructables.com/id/Brompton-pannier-adapter/