As Gaurav said above,
I have a big military bag with everything in the bag (multiple compartments, yes also the jack). I always secured it with bungie ropes (or how do you call these things) up to last week when I made a big jump.
I realised that it might be not strong enough if you start rolling or in worse case scenario a forward flip. Don't underestimate the g-forces that occurs during an accident. (Imagine your jack flying on to your head)
After this incident, I bought tie down straps and secure the bag firmly. Yes, it takes more time to secure, but we are talking about your life.