Depends what your armour and back pack are made of?
My chest armour is made from exercise mat, and back pack from foam board. The first time anybody got close and personal, the two came apart.
I've since Made a frame out of 22mm plastic pipe, and covered with foam board and foamies, which I've bolted through the exercise mat back armour. If you go this route, use large washers to spread the load.
Good luck.