[utopia]

I got one of these last year and I notice that my local Aldi has some more in now.
Made from tough, heavy, reinforced pvc-like material with a reliably waterproof, roll-top closure, they make a nice, large, cylindrical pack for strapping on to the pillion seat.
On the recent coast to coast run, I was carrying mine with tent, airbed and sleeping bag all inside. It survived the downpour over Dartmoor without even a hint of dampness.
The clincher is that they're only a tenner, so worth having even if just for occasional use.
Downside perhaps is that they only seem to make them in dark blue or yellow.

While you're in there, they have cans of GT85 for £1.99 too.