Using the mineral board roughly cut and temporarily spaced off the bulkheads. That way you could get a sense of the amount of heat you'll be dealing with, adjust the backboard, etc. I think it is the layer(s) or air that do the real cooling. My guess is the hot spots would be to port and forward of the stove body. The bottom has some good air space and may have firebricks? Any firebricks in the sides? The tile looks great.

How is your Hurricane system on burning up amp/hrs? Those hydronic systems look like the best way to heat a boat to me and supplying the domestic just makes them that more attractive.