I almost panicked - then I remembered that I had my phone in my pocket. But I just couldn't call for help. Maybe I'm just as stubborn as John when it comes to doing things on my own, and the odds of me getting a signal out in the pasture and inside the box were slim. So with renewed vigor and less and less feeling in my toes, I began furiously kicking the edges of the door. It didn't budge. Next I tried several full on body slams (a great way to release frustration, incidentally). I stopped and checked the door again but there was no movement. I thought I might actually have to try the phone at this point. Before giving up, I gave the bottom of the door one more kick and tried the handle. With a dramatic sucking noise it opened!
So, no matter what John says about efficiency, that's the last time I'm working in there with the door fully shut. Other than that one issue, the walk-in is a perfect fit for our operation, with room to expand.