Definitely ditch the seatpost rack.

I've been carrying my bookbag like this for the last two years. Just a regular rack bolted to the frame and two bungee cords holding the bags on (one along the length, the other along the width). The bags weigh in at 22lbs or so (depending on which classes I have). Takes only a few seconds to undo the bungees and I have not had it come loose or anything despite having run into a tree, ditched into a field, and hit numerous potholes. That milk crate looks pretty cool, though. I'll have to try that out.