I finally bought a luggage rack and a large trunk from JC Whitney for my S40. I have been needing lockable storage, and this seemed the least expensive way of getting it done. I looked at the Leatherlyke bags, but just couldn't go the cost. This setup cost me about $170 for the rack and trunk, and about $20 for some cutoff tools because . . . the trunk wouldn't fit behind the stock sissy bar. The bar angled back too much and I was left with only about 1 1/2 inches to mount the trunk plate.
Here is the bike with the case mounted:
Here is the plate with the cut off sissy bar:
Here is the trunk from the front with the seat re-mounted:
Cutting off the sissy bar was a bit of an adventure, as it's solid chrome steel, about 5/8 inch square. I should have cut it lower, and I probably will trim it down after I get some more cutoff disks. I went through several getting to this point.
The trunk removes with the key, and I may drill it and put in some bolts to back up the latch. It will be holding my laptop, after all. I don't plan to remove it often.
Here's the link to the trunk:
http://www.jcwhitney.com/autoparts/ItemBrowse/c-10111/s-10101/p-100000346517/...I got the luggage rack from Oneida Suzuki.