In the 70's I built a combo reloading room, shelter and gun safe room into a below ground basement corner. Heavyweight blocks, vertical re-bars into a new footing, horizontal mesh wire, filled cores, poured ceiling. Basic rectangular shape with a similar guard wall on left side with a second door door 90 degrees to first. Hardened drill rods for internal hinge pins, 1" etc; this is the tough way to go. You will need a milling machine and Buzz Box welder. Browning makes a safe door unit for building into new construction. Easy and simple and works first time. When you build yours, ABSOLUTELY get/ build in integral de-humidification with automatic drain and some (electric ) heat, unless of course you like rusty firearms. Overbore