I had a walk-in safe where I used to work. Thing scared me to death.
I had seen it constructed and I knew that if I ever got trapped in it, it would be my tomb. It was in a somewhat remote part of the building and I generally worked by myself.
I put in emergency lights, a phone, and both a rechargeable and a regular flashlight. I practiced unlocking the door from the backside. Anytime I opened the door, I threw the bolts so it couldn’t close and lock on me.
The door opened toward the outside, which wasn’t too bad since I had no intention of ever closing the door while I was inside. For the purpose you describe, I’d consider an inside opening door to be absolutely essential.
I forget where the door came from – some place in Georgia I believe. It was rated for the storage of Top Secret material (not that I had any) and cost several thousand dollars, so it’s not what you’re looking for anyway.
I hope you’ve got some sort of ventilation set-up. Personally, I’d also look into a second access door, maybe 18 inches square – probably made of metal plate bolted inside the vault (with a wrench nearby). I’d also throw some noisemakers in there in case you do get trapped – whistles, air horns, etc.