I have tries all kinds of bags on my passenger seat and never could fine just the right one that would easily strap down, and allow me to open & close the tourpack without it getting in the way... Most often they are too wide and get caught on the arm rests or too deep and you have to keep fighting with it to keep it from being pinched by the lid on closing.
When traveling I always check out the local dealerships for the supply of bags they have and try them on the bikes in the showroom, to no avail... As luck would have it, (and you know how you never find anything when you're looking), just yesterday while roaming around the Toronto Motorcycle Supershow I think I found the perfect bag!
It measures 15"w X 12"h X 10"d and apparently is waterproof! Appears to hold enough for what I want and prior to purchasing it the fellow at the booth allowed me to take it away to another booth to try it on an Ultra. This of course has not been tested by myself on the road yet, but it is the best fitting bag that I've found.
The best part is that with the "show deal" it was a great buy at $39.00 tax in!
Certainly not expensive, but I think it'll do me just fine for what I need it for.
Unfortunately there is no label on it, so I don't know what the make is... The company was from Montreal and they just travel around to different shows displaying all their various bags and other products.
I believe this size of bag will work for you and can only suggest to visit a few shows going on at this time of year and don't walk past the little booths that you think won't have anything your interested in. But whatever you do, don't go out looking for a bag, 'cause you may never find it when you're looking.
Good luck!