Er, I remember reading a tutorial for it once. You make a box. Make another box around it (bigger, ofcourse, not by much). The bigger box should be hollow; it should be a Func_Illusionary assigned the the texture "{blue" (in HL.wad). Only the inside of the hallow box's faces should be a pure black texture. Don't qoute me on this; it may be inaccurate.