This is the one I'm thinking about buying and hoping will fit. It attaches with bungees and our bikes have a pair of hooks on them. I haven't been able to find it locally though to take a look at one in person. I'm thinking about buying another rear seat off ebay and permanently attaching a tail bag to it and just swapping them out as I need to. Good luck.
Oxford Sport tailbag. Remove the bungies, fold the flaps under and just use the wide velcro strap to attach it to the pillion seat. $45. Has a tendency slide forward a bit. Haven't sorted that out yet.
EDIT: oh, it's not smallish, but I have no problems finding a use for the space.
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