Notes:

A weathered and a new unpainted box have been added to the test.

Conclusion:

The most important factor to reduce heat is color of nest box. On sunny hot days light colored heat shields can be added to light colored boxes to reduce temperatures another 2°F-3°F.

Recommendation:

Use a light colored paint and/or heat shields to protect your nest boxes.

Note:

There was a question about ventilation in these boxes. There is a 1/2in high 4in long vent in the front and also a pair of vents in the upper back. See photos below.