First, you should choose a professionally-manufactured light box fixture, rather than trying to make one yourself. Select a light box from a reputable source and consider major manufacturers.
In selecting a light box, you will want to choose a fluorescent light source over an incandescent one. Make certain to pick a light box that has features to protect your eyes, including screens or diffusers that eliminate UV rays that can cause cancer, damage to your retina, and other harmful effects.
It is most important to select a light box with the appropriate light intensity. A full-spectrum light, which includes all colors of light, is unnecessary. However, the intensity of the light should be 10,000 lux, and you should ensure that the light box you choose meets this standard. One lux is the equivalent of one candle burning in a completely darkened room. For comparison, the sun on a bright day is 100,000 lux. Light boxes with less intensity than 10,000 lux may be ineffective and a waste of money.
Unfortunately, light boxes are not universally covered by health insurance and may cost from $200 to $500. You may want to ask your health care provider if they have specific recommendations for a reputable source. Alternatively, exposure to sunlight - even on a cloudy or rainy day - is free and may be just as effective.