Make sure your hinges are either welded-pin or have a set-screw to prevent the pin from being removed (Only an issue if the
door opens out).
Grills in the
door need to be secure, and ideally should have heavy steel of some sort to prevent someone gaining access. Just the other day I had the idea to zip-tie a box fan to the inside of the grill on the
door to draw air in, but I couldn't fish the ties through so I went to take the grill off. The idiots who installed it put it on backwards and for the last 18 years anyone with a Phillips screwdriver could have just taken out four screws on the outside and climbed through.
One local operator I know swears by a multipoint
lock system with a single keyway in the center of the
door and bolts on all four sides. I haven't looked at it carefully but I wouldn't use a single keyway system unless it was something extremely pick-resistant like a Medeco
lock.
Right now I have a commercial grade Schlage lever
lock and two deadbolts, both much higher quality than household stuff from Home Depot. It was expensive but it would take a non-pro an extremely long time to get in. One deadbolt is double-cylinder in case they gain access through the roof, then they can't go out the
door.
I haven't been able to find a proximity entry knob, something where I could carry a fob that would let it unlock when I'm within 10 feet of the
door. I like Schlage's household keypad entry, but I wouldn't use it where I am.