I'd suggest a set of Wald folding baskets- fold them up when parked, unfold when shopping or carrying your backpack. That is gonna be one of the least cost options.

I'd also suggest that you experiment with flash drives and 'cloud accounts' like Google Drive and leave the laptop at home. I found that I was less prone to waste time on social media sites if I used the school's computers... and less time worrying about my laptop being damaged while riding.