thanks All for the replies!

I tested out bungee cording my laptop case to my rack and it works fine. It will do for now. I like the Topeak trunk bag, but since I don't have a Topeak rack, I would have had to strap/bungee cord the thing to my rack so I figured using my existing laptop case is good enough.

I also placed an order for a Pelican laptop case. it will hold the laptop, but not any accessories (power adapter, etc.). i would like to use this because it is waterproof and has some impact protection.