I've never seen a "top box" on a bonnie before... and typically luggage solutions are pretty expensive. There are these:
http://newbonneville.com/html/luggage.html ... but that is quite a price including the rails.
For an extra helmet, you can buy a cargo net to strap one down to the back of your seat (plenty of room for that).
Great Bay Motorcycles has some good products available (
http://www.greatbaymotorcycles.com/c...?id=Bonneville), but again, market price is what it is and they are not "hard" as you are looking for.
Depending on what you need, a tank bag might solve
some of your storage issues. Good luck!
