Here in my town in Sweden, where a large percentage of the population gets around by bicycle, a lot of people have sprung mouse-trap style racks, similar to but nicer than the common Pletscher I once used in the USA. They'll use the trap to confidently attach expensive looking European handbags, regardless of the sex of the rider...

A big issue for you is going to be: What happens if it rains? I once kind of ruined some engineering textbooks when using a cheap backpack in the rain. I used them through the end of the semester, but they were wavy and crinkly. Today I use a very water repellent messenger bag that I bought ten years ago. This reminds me that it's time to replace the broken buckles...