Commuting by SS/FG

I have a very large carradice saddlebag supported by the rack (while tie to the saddle naturally), it hold my 15" laptop with room to spare, perfectly doable.

Get a longflap version though, extremely useful when you're carrying large object.

I got me a Carradice Nelson Longflap to be able to carry my ThinkPad on my road bike, but I found out that I dislike having so much weight on the bike and I take a backpack on the days I need to haul the computer.

But the Longflap really isn't overkill for just a change of clothes, towels, tools, spares etc and you have some extra space available for little groceries etc.

It's a bit slow to install/deinstall to the saddle loops, but it helps a little if you change the straps to toestraps. Leave the toestraps long; looks goofy but helps a lot when installing especially if you don't have a rack.