I like the Dowco/Iron Rider stuff.

 

I have this T Bag, used it on my Goldwings, BMW RT and now my Ultra LTD.

http://tbags.com/backseat/dresser-backseat-bag.html

 

I also have this bag. The shoulder straps make it a great replacement for my roll-aboard when I fly to wherever my bike is on the mainland.

 

I highly recommend something round or oval. I have had the square sided units damage speakers on the tour pack before.

 

I searched this about a year ago and my preference was something that did not utilize the luggage rack on the tour pack to secure it, even though it was on the back seat. I still wanted easy access to my tour pack... that and I don't have a tour rack mounted anymore.

I ended up buying a VUZ Moto bag that I love. I can use the straps it comes with to tie it down to the bracket underneath the tour pack and it's completely waterproof. To clean it, I just throw it under a hose/shower and wipe it down, done. Comes with 4 tie down straps, lots of D-Rings to attach things to and a shoulder strap.

They since have came out with a duffle bag with the same benefits that I would give a serious thought.

https://www.vuzmoto.com/products/vuz...le-luggage-45l