We DO NOT recommend adding an Ash opening / dump to any wood fired oven.
You will spend more time trying to make a proper ash opening than you will removing ash for the first 10-15 years you own you wood-fired oven. It only takes seconds to remove ash w/ a small ash shovel (I simply push ash to the side / rear of the oven with our pizza oven brush / scraper then lift the ash out of the oven with our pizza peel - like a shovel).
If you have an ash opening, you will need a custom cover to keep the cold air from entering the oven (this area will always be cooler than the rest of the hot oven).
Unless you are a professional wood-fired baker pumping out loaf after loaf of bread (day after day after day) you won’t need an ash dump.