Sealed Enclosure in Sunlight
A sealed box outdoors settles where what goes in equals what comes out:
P + α·G·Asun = h·A·(Ts − Tair) + ε·σ·A·(Ts⁴ − Tsur⁴)
The surprise, every time, is that the sun is usually the bigger heat source — not the electronics. A box this size shows about 0.06 m² to an overhead sun; at 1000 W/m² a black finish takes in nearly 60 W of it. No amount of internal cleverness competes with that, which makes the finish the first design decision and shade the second.
α and ε are independent, and that is the trick. Solar absorptance is a short-wave property, emittance a long-wave one, so a white paint can take in very little and still radiate a great deal. Both are inputs here because they belong to the coating you are going to specify, and its datasheet is more accurate than any generic table.
The box
The day
What the fixes are worth
A steady-state lumped balance: one skin temperature, one internal air temperature, still or uniformly windy. It does not model hot spots on a board, the conduction path into whatever the box is bolted to, thermal mass and the fact that the worst hour is not the whole day, internal fans or heat pipes, or the difference between a component case and the air around it. Sky temperature is Swinbank's clear-sky correlation; α and ε are yours to take from a coating datasheet. A helper, not a substitute for engineering judgment.