The "lightbox" is the light source of the vidéoprojecteur.
I've choosen to use a HQI lamp (mercury vapor). Benefits are a very white light (movies will end with a perfect color), and it's a ponctual light source, perfect to align the focal of all the optical elements. Plus, this lamp has a life expectancy of 10.000 hours, way more than the ones from the market's VPs ; and when it dies, it costs only a few dozen bucks to replace.
Drawbacks : it heats as hell! So you have to make sure its support will handle the high temperature, and a sufficient fresh air flow.
Cutting the support in 1.5mm thick sheet metal... Top of the picture, the reflector that will come behind the lamp.
Folding the sheet...
Assembling the lightbox : lamp supports are made out of ceramic, and one of them is mounted loose to handle the lamp dilatation due to the temperature.