Thanks Waldo. I understand what you mean about building a bed of coals. Originally I was thinking about using a steel plate resting on the remaining grates. I also considered using a layer of firebrick resting on the remaining grates. I had planned to leave a gap at the front of the firebox that would allow ashes to be dragged so they would drop into the ash pit. It's impossible to make anything that is one piece because the door opening is only 14.5 inches (16 inches corner to corner) and the firebox is 17.5X21 inches. So a layer of firebrick would be cheap and simple to custom fit to the firebox.
I had a Kerr Scottsman wood furnace once, It was not a boiler but had a fan and ductwork. It was just a firebox with no ash pit and they insisted on not using firebrick in the combustion chamber. My original thinking was based on the fact that the Scottsman furnace worked good.
I have the choice of the combustion air entering from slots in the the door and/or entering from the back of the ash pit under the fire. The draft control on the back could be motorized and tied to a thermostat. If the rear draft control was used the air would travel under the firebrick to the slot at the front and over the fire. There are also slots around the outside edge of the combustion chamber where air can enter although they might get blocked if I put in firebrick. So, you don't think I need to line the outside edges to keep the fire away from the water jacket?