Over the years, on a variety of big bikes (for touring), I've had good results from Saddlemen products.
I've used this one on the rear seat, but had to put protective film material on the top of the OEM saddlebags to avoid possible scratching. You also have to be fairly creative with the type of straps or bungees you use to secure it to the OEM passenger handrail.
I'll also use their rack-mountable barrel bag on my B.
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