Ace Durahide Sissybar Bag
I have this sissybar bag by Ace, the smaller size. I mounted it to the back of my sissybar pad with two well-nuts. I can remove it easily in about 2 minutes if I need to pack something on my rack. The two straps are a little fussy. They also sell a Saddlemen "Cruisn" sissybar bag with velcro, that might be more convenient. There is a wide selection from this website and they had fast service.
I removed the conch from my bag and replaced it with a Triumph patent badge from a keychain ($7/keychain). Here's a picture of my bag mounted:
