I've got a big Nelson Rigg tailbag -
https://www.nelsonrigg.com/store/product.aspx?pid=446
Secures very easily and stably (one strap on each passenger peg, and then one strap that I usually cross under the tail to the opposite cargo hook). Once it's there I tend to forget it's there - weight doesn't shift at all.
I commute with it - usually has a laptop and my 9x12 leather billfold/portfolio thing, my shoes, my lunch and a coffee mug with tons of space left over to drop off dry cleaning and such. Un-expanded, I can put my helmet in it and I've never actually expanded it.
I've also got a pair of their saddlebags, but haven't used them yet.