I'm struggling with the right answer on this one. I can't afford the circuitry to get too cold, there's not much room for insulation, and the circuitboard is long and thin, which doesn't help with heat retention.
So, I'm thinking about painting the fuselage black. I found some data (below) from this pdf on thermal management of spacecraft, and it shows the temperature of a sphere vs altitude above earth for a black body, gold, and a white body.
If we assume that the aircraft gets up to 35Km, it looks like a perfect spherical black body will be at 60-80 Celcius, and a white body will be around -20 to 0 Celcius. If the plane has a triangular fuselage, spins on ascent, only one side faces the sun, and the angle of incidence to the sun is ~50% less favorable than a sphere (guesstimate), then I'd expect average surface temperatures to be somewhere between 10 and 30 Celcius - great for the electronics inside.
On descent, even if one surface is facing up, i expect windflow will be sufficient to keep the surface cool. I'll post pictures of the aircraft shortly.