Ambient temperature/wind chill - Keep your Caja China out of the wind as much as possible. Set up on the "lee side" of the house or garage, or throw together a couple of sheets of plywood (at a safe distance) to block the wind. Cooking in extremely cold weather is just going to take longer, it can't be helped, so plan ahead for it. The wind-deflector is a vital accessory for cold and breezy days.
Ash build-up - Ashes are an extremely effective insulator. Even a 1/2 inch layer, between your coals and the pan, can cut the amount of heat going into the box drastically. La Caja China's instructions call for removing ashes roughly every three hours, by lifting the lid and dumping.
Ash is a great insulator and an inch thick layer can easily keep your Caja China 30-50 degrees below your goal temp (225-250d). The catch-22 is that you need to get rid of the ashes, but you want to avoid removing the ash pan, if at all possible.
Do this by lifting the coal grate and turning it sideways across one end of the ash pan, then use a large metal dust pan (square edge) and scoop out as much ash as possible.
Next, slide the coal grate to the other end, and remove the rest of the ashes the same way, then set the lid back in place. You’ll see the temp jump 30-50 degrees when you do this.
Assembly - The most important piece of advice on assembling your La Caja China is this – DO NOT TIGHTEN THE BOLTS UNTIL FULLY ASSEMBLED. Leaving them a little less than finger-tight helps in getting all of the holes lined up correctly.