Because of edge effects, the heat leakage through the insulated back-of-the-plate is difficult to calculate. It would be better to measure it; but that measurement must discount the heat transfer through the front side of the plate. The .421 W which I estimate for leakage is less than the downward-facing convection (.525 W) for a plate at 5 K higher than ambient temperature. Because the test surface of the plate is rough, I don't have confidence in convection calculations based on smooth plate measurements; and assuming what I am trying to measure means that I wouldn't be able to test the assertion that surface roughness has no effect on downward natural convection.
So I created insulation for the (rough) front surface of the plate. Constructed using the same materials and techniques as the backside insulation, its leakage should be roughly the same as the backside.
While cutting the extruded polystyrene foam I noticed that the board crowned in the center. So I arranged the cut strips so that their centers pressed into the plate edges while the duct tape tensions their ends into corners. It maintains a snug fit and doesn't need tape to seal it. Because of the bevel, the backside foam is more complicated to cut; but I may at some point rebuild it using the same technique.
Because the expected downward-facing convection is close in magnitude to the leakage, the fully-insulated measurements should be made both level and at some inclinations (convection increases rapidly as the plate is tilted).
