Lighting facilities sold commercially are very expensive. Since I couldn't afford it, and I'd hate to move away from my beloved Cape Town, I have asked for advise from other members on the list. Using this box daily for the last three months has changed my life. If you want to build and use a light box as per my specifications, - you do it at your own risk, I don't want to receive any lawyer's notes.
Light therapy uses light emitted from 'full-spectrum' lights.
A 10,000 lux light source at a range of 1 meters for about 30mins a day, preferably in the morning seems to be the recommendation. Brian's calculations suggest that this means that if the illuminated area is about 4 sq. A 5ft fluorescent tube produces about 5000 lumens so you would need 8 of them together with their fittings. They need to be mounted close together to provide the concentrated source you need and they are going to take about 500 watts of electricity. You should not need a diffuser with ordinary fluorescents, it would be mostly aesthetic.