not sure I'd put a macbook in bag attached to any part of the bike. The shock of the road and bunny hops may crack the screen.

I have the apidura/rapha bag and it's 11l, will fit commuting stuff just fine but no where near space for laptop.

For you, I'd suggest a 20-30l slim backpack with some venting padding on the back.
I like this and like them
Timbuk2 Spire
https://zitseng.com/archives/14466

and
northface Kaban Transit (factory outlet for $37 score)
The North Face Kaban Transit Backpack Review 2016 ? RMK Outdoors
https://www.thenorthface.com/shop/kaban-transit#hero=0