I use the first version of the Direct Action “Dust” - https://us.directactiongear.com/dust-backpack#BLK

My brother liked it so much he bought the mkII.

It is on the small side, but fits a Dell XPS 13 or even a 15” MBP (but definitely not any bigger). Helps me keep the amount of crap I carry way down, and is super tough. Straps are wide and comfy, and the back has padding that helps the back breathe a little.

I like poole.py: https://bitbucket.org/obensonne/poole

Simple and to the point. Probably not good for larger sites, but it works for small ones.