The idea is to put a piece of very hard material between the bad guy and the lock. That makes matters much more difficult. Also a piece of toughened glass which will go to dust if drilled. Relockers are attached to the glass plate and fire as soon as it is drilled and before the drill gets to touch the lock face.

I can quite sympathise with your idea of using a door lock, and why you want to make things cheaply as the safe will have almost no value when it is finished. On the other side, a door lock like that is certainly a weak point in the system. Having said that, how often are safes accessed by a criminal picking a lock?