I think with downdraft and wood on top of the combustion chamber, it has to be made like an arch. I don´t se how a ceramic blanket would hold up. However all cleanout doors on my boiler are dressed with similar blankets on the inside. They act both as insulation and door seals.
Dan, my boiler has that flap, but I don´t use it much at lightup. I might open a small gap to vent smoke out the chimney at a hot refill.
I do recomend a helper fan on the chimney. It makes the downdraft lightup procedure so much easier at a cold start.