Hey Mash, don't want to burst your bubble but the Rebel's tank is too narrow to fit a bag of your dimensions. I was looking for a bag for my ipad Air
!, and couldn't find a good looking one that'll fit right.
Some bags will fit the iPad or your 13" MBP OFF
the tank, but due to the narrowness and slope of the tank it will not work. So I've grown to love wearing a backpack that will also allow me to carry my electronics, work out gears, shoes, and everything else. You have to take off and carry the tank bag anyway so why not get a backpack that'll do more.
Now, for short errands or cruise days, I either use no bags at all, a small tank bag or my larger tank bag on longer trips with my backpack.
I've bought and returned many tank bags, but the ones I stuck with are made from Chase harper, here are some images to show it's dimensions and scale
The first image shows the Chase Harper 950M
(same dimension and volume as the 650M, 787cubic inches)
The second image shows same the Chase Harper 950M (expanded to 1125cubic inches)
The third, fourth, and fifth images show the Chase Harper 450M
(about 250 cubic in.)