I don't care for backpacks either. Most of them are oversized, bulky and uncomfortable.
I did find this low-profile backpack:
https://www.betabrand.com/mens/more/...-backpack-gray
They used to have a version called "Flashback" and it is made of reflective material. That's the one I got, last year (knowing that once it warmed up, I was not going to want this thing on my back).
So, I went to Ortlieb's website, navigated to the "Replacement Parts" page and ordered a full Ortlieb mounting system for my backpack. I pull the straps around to the back and secure them together with a small strap, hang the bag on my rack and it is not going anywhere.