Hi John.
I use the Motodry Roll Bag.
Very roomy, strongly built and fits snugly in the concave curve of the rack.
I added a heavy-duty cardboard liner (cut from a 30cm diameter cardboard drum) so the bag always holds its shape. It also has a storm cover tucked in a back pocket.
You might prefer this one - slightly bigger but a colour scheme to match you bike

.
You can see them
here at the Aussie distributor's website. I got mine from an online store in Aussie.