The Osprey Transporter 40l with Rok straps, bike lock and padlocked zipper
Osprey - USA
Cost - $100 AUD
This is a superb bag. It's not a drybag and you don't need one. Unless you plan on swimming down the river with your motorbike. Which I don't.
You get a lot of bag for the price. It's fairly water resistant and is made of a heavy waxed canvas. It has two strong vertical carry handles at the top of the bag that make lifting it on and off the bike a piece of cake. The two horizontal handles make carrying it to your campsite even easier 👌
Need to travel further? Pull out the backpack straps and clip them in. This leaves your other two hands free to carry your other panniers/gear. It's surprisingly comfy and I wouldn't have a problem trekking with it.
It sits secure on a top rack thanks to its flat re-enforced bottom. Access is from the top so you don't have to take it off the bike to get inside.
It fits all my camp gear. It also happens to be the perfect size for check-in plane luggage as well. So I'll use it on my trips to see my girlfriend in Malaysia.
You could go for the equivalent more expensive and lower quality North Face Base Camp Duffel or any roll top that is a pain to get into and carry. But seriously, why would you?
Waterproof Bag Cover 40L
Cost - $10 AUD
Stuff it in the pocket used for the backpack straps and bob's your uncle or leave it on there permanently.