Help with finding luggage or new backpack that I can mount?

Hi,
I commute about 50 miles each way to work. I carry a pretty large backpack with me that has my laptop and accessories as well as gym clothes. I'm looking for a way to transport the bag to work on my FZ-10 without wearing it while riding. I picked up a set of the SW-Motech bags from France, but that was a big mistake as my laptop just won't fit in the bags and honestly it's just not a good solution for this particular scenario. The backpack is large, about 19"D x 11"W x 16"H and weighs, I don't know, around fifteen pounds when packed.

The SHAD side cases would do what I want but they're just so damn big. Plus I already spent a bunch of money on the SW-Motech bags, so I'm not ready to pull the trigger on another set of saddlebags yet. I don't think this backpack would fit on the tank so that's mostly out, so I have a few choices left that I'm aware of - Get a top rack for the rear and a small case for the backpack, or get a new backpack that'll mount to the top rack, or some other combination that I'm missing.

I wasn't even aware that I could mount bags to a top rack using the SW-Motech quick release system designed for the tank, so rather than spend hours reading about every dang combination I figured I'd ask you guys - Can you recommend some kind of backpack that I could mount to the bike, or a good top case that I could use to transport my existing backpack for my daily commute?