Ed, I put mine in that same locker just like you, but I only have one vent, coming thru the louverd door.
When I talked to my AC guy, he wanted the 16k unit and I wanted the 14k unit. We both felt the 12k unit might be too small. When he went to order the 14, it had been discontinued, so we both decided to go with the 16. Wish I had gone with the 12k. Yes, it would have struggled more on a hot, sunny day, but would have been the better unit to have once the sun went down. A comfortable, good night's sleep is very important to me.