anyone who used a rucksack as hold/hand baggage and knows if you must put a lock?

I guess this depends on the airlines as well? I used the China Eastern for a domestic trip from Beijing to Nanjing and these is no such rules for carry-on luggage. The checked luggage surely needs to be well packed, but for the carry-on luggage that is OK. I saw Chinese passenger board with plastic bag in hand. Maybe you need a confirm with your airline to see if they have special rules on that.