You don't like floppy rack bags or panniers, so your solution is to buy one of one and two of the other.
Nowt so queer as folk, as they say in your part of the world.
Not so fast.........................I picked them up tonight, the Altura bag is immeasurably better than the cheap ones I've had from Amazon. It's double stitched at crucial points, double walled and within these walls (on three sides) is a stiff piece of plastic, bingo! Sturdy & stable bag.
Also, it attaches from all 4 corners at the base ,via velcro straps, around the rack, the cheap ones needed several cable ties to keep them (half ) stable as they attached only from one central point at each end. The bags are £30 new, this is like new. The panniers are fine, Airedale, never had any experience of them but they will do for me. £4.99 all in. 
