not overkill. I make medieval armor and have a LOT of experience with making things absorb impacts..
Double boxing is a KEY element, the other is restricting movement.
The idea double box is that the outer box takes all of the destructive impacts and the inner box hold the internal parts isolated from those shocks.
The most damage occurs when the inertia of a 25+ piece of iron starts moving and then stops suddenly, any slack or "wiggle room" will let that inertia build up and cause damage when it stops moving.
(Kinda like the old "when jumping off of a building it is not the fall that kills you ,it is the sudden stop at the end")