Hi smeyn,
When we toured in Tassie we had 2 Ortlieb panniers. On the top of the rack we had a plastic box with a lid. it was a toolbox from Bunnings. It was held with cable ties through small holes in the bottom. On a later tour we had a soft bag like they sell for putting half a dozen cans in. It was for a bike and came from LBS. I think the box was better but things could rattle.
Next I bought one of those square section plastic kitchen tidy bins and divided it in four lengthwise. A quarter was cable tied on each side behind the seat. Slots were cut for straps.
A good sized drysack of gear could be fitted each side into these carriers.
We had a couple of spare tires 20" and 18" that sat on top around the box on the rack. We used to hang two more drysacks from them. One each side.
The self inflating mat went on top. Lengthwise usually but we'd put it crosswise if there was a tailwind.
There were two of us each set up this way and we had enough gear and clothes etc for or month's meanderings.
Heres a shot. Had a bit extra hanging round but you get the idea

John