We discovered from our own experience that it is usually not cost-effective to build your own lightbox, especially a compact and portable lightbox. Aside from the fact that it can be risky for anyone but a trained electrician to build and wire an electrical appliance, prices for quality lightbox components purchased in small quantities are very high. The special tubes and ballasts used in lightboxes are not standard electrical supply house items, and calibrating a lightbox properly requires special instrumentation and testing facilities.
However, if you have the skills and are eager to do it yourself, here are some guidelines. To protect your eyes, lightbox tubes should not emit any ultraviolet light. They should be powered by electronic ballasts, to ensure that the tubes will never flicker or make a buzzing noise. The "color temperature" of the tubes determines the aesthetic appeal of the light; use tubes with at least a 4000K rating (higher ratings are generally more pleasing and "natural", although they are not essential to producing a therapeutic result). It is necessary that the box deliver light of sufficient intensity - 10,000 lux - and you should make sure that you will not have to sit uncomfortably close to it to obtain this therapeutic dose of light in a reasonable time span (typically 20-30 minutes/day).
If you suffer from seasonal depression or problems regulating your sleep/wake cycle, the purchase of a lightbox will more than pay for itself in terms of increased productivity and sense of well-being. You can incorporate light treatment into your daily routine.