I'd just bungee it to the pillion pad as has been suggested. 2 of them crossed over will be fine.
A hard topcase - might be hard to find one in your size, being 3' long. They're lockable, and waterproof. Some are helmet-sized, so when you arrive you can take your bag out and put your helmet in to save lugging it around.
A rear rack and soft bag. I've used this before, a Ventura Euro Rally bag (huge, expandable). Can be put on the rack either way around, depending on whether you have a pillion or want to keep the weight forward. Easy to lift off when you arrive. I've carried an esky full of beers/ice on such a rack once or twice before - well over it's rated carrying capacity and made the bike handle real weird, but worked really well.
Panniers - either hard or soft. Hard ones are lockable and waterproof, soft ones are light and come off easy. I've used soft, expandable ones before with integrated rain covers, never had a problem (same as the pic below, actually). Have carried 4L tins of paint in them without a problem. The weight is down low, which is nice. They make the bike wide, which can impact your ability to lane-split. General advice is not to have bags wider than your mirrors for that exact reason.
Tank bag. Either a magnetic one (if you have a metal tank) or an attached one. Very secure, and good for cameras, laptops, etc. Sits on the most stable part of the bike, less vibrations, etc. Good for touring because you can put a map under the top cover of most bags. Can get expandable ones that hold a bunch of stuff. Weight is nice and central on the bike, easy, predictable handling. I've never used one, but wanted one of the ring-attachment ones for many years.
When I was travelling on the bike for work, I'd typically have the laptop in a backpack on my back, or in a satchel thrown across my back. I'd stick the laptop itself in an upside-down plastic bag inside the backpack in case it rained. If it was an extended trip, I'd have clothes in a bag on the rack / panniers / bag strapped to the seat.
Given what you're after, I'd keep it simple and just strap the back to the pillion seat and be done with it. Unless you're planning on doing some touring, which is where the investment in other carrying systems pay dividends. Although I do like that tail pack on the ZX-6R above - that's clean.