While obviously a less expensive version of the high dollar bags sold by a brand that rhymes with Briega, the Rhinowalk 20L offers a lot of storage and usable features for a very small price tag. I picked this up from Amazon for under $50. You can even get a model with yellow panels instead of black.
I put the bag through about 3 hours of riding in the rain a few weeks ago, as well as almost 1,300 miles of riding over 5 days. It definitely outperformed it's small price tag. For me, it held enough clothes and supplies for the trip, including a lightweight sleeping bag, and camp mats. I did have to lash on an extra bag to carry my shoes since I prefer not to walk around like Frankenstein's Monster while off the bike. If the weather would have been more consistent, I could have fit everything but when dealing with days that may have a 28 degree temperature swing, you have to be prepared!
I did see some strain on the extra lashing loops, but I had the external bag on there very tight. Other than that, it performed flawlessly. It took me a bit to figure out the best spots for the loop straps but once I found that, it's easy to remove and mount. I realize my bike isn't the target audience but with the straps, you should be able to mount this on any motorcycle with some creativity. I do think the straps could be a bit longer and I could see where a bike with a thicker seat may pose a challenge.