The NEMA enclosure should have an IP (Ingress Protection) rating sufficient to keep the enclosed equipment safe from the elements (search for IP rating in USAT’s common questions for more info).
Glass polycarbonate is the preferred material for the NEMA enclosure. Unless in areas with high security or vandalism risks, glass polycarbonate enclosure are sufficiently durable and conduct less heat to the enclosed equipment then metal enclosures.
The enclosure should also be vented. Venting does allow excess heat to be passively removed, but also facilitates pressure equalization in changing weather conditions. Venting can be accommodated while still maintaining the units IP rating by the use of labyrinthine vents.
Generally speaking active venting is not suggested. Active venting uses fans to force air out of the box. Fans have the disadvantage of drawing environmental elements into the box including dust, airborne particulates, humidity, and even water. Fans also draw more electricity and can fail over time. If they must be used, they should be thermostatically controlled fans and highly energy efficient.