I have the
Leatherlyke cruiser bags, a bit pricey at $430, but lockable and durable. Not to mention they were easy to mount and look, IMO, rather classy:
Either side can hold two gallons of milk, or a 10# bag of potatos, or a 5-liter box of wine, or a classic leather motorcycle jacket, or....
As for the laptop, since you'll be taking it with you from the bike, why not get a rolling laptop case and add a sissybar mount:
(click=video)If you want the box instead, hoese also makes a topcase to match the cruiser bags -- I would lose the pillion seat, flatten the sissybar, and fabricate brackets to hold the box where the pillion should be to also add a driver's backrest.
With the S40 you could probably make brackets to mount to the siderails to carry the box over the pillion.