Here is what I use when I need some cargo capacity...these old Shoei hard bags...they bolt right onto a cb750, and probably a lot of other similar bikes too...certainly no longer available new...but I think there are a lot of 'em out there and I think they look great (or would if I painted 'em to match), so keep your eyes open on ebay, craigslist, and at the swap meets
edit; oops, pictures won't work for some reason, if anyone really wants to see them, I will post a link to my project thread