Never been a fan of back packs/rucksacks, because they used to make my back get too warm.
Have got a magnetic tank bag but never felt that comfortable when using it, it always felt cumbersome and in the way. Not that it matters now as i just put a Baglux tank cover on it so the magnets won't work.
As my bike is mainly a commuter, the top box is a necessity for me. If it was ever empty it would fit 2 crash helmets in quite comfortably, but it mainly consists of light tools, several gloves for different weathers, waterproof trousers, bike manual and a carrier bag for stuff for work. There's a few bungees chucked in there just in case the seat needs to get used for anything that doesn't quite fit inside the box, or is too heavy, i won't overload it after my Renntec sports rack snapped clean in half on me.
I had 3 metal plates made, to go inside and outside the box, and then firmly bolt it to the other plate underneath the back rack, so it's solid in position and also is positioned far back enough to give a pillion plenty of room.
Stuck a 1 inch strip of reflective tape around it so even the Mr Magoo's on the road can spot it at night time.
My bike chain stays firmly planted under my seat for obvious safety reasons.