The form of light-box presupposes the choice of illuminant, and in this there is a wide range.
A light-box is basically a frosted pane of glass with a light beneath it to produce soft, even, non-glaring illumination.
Having the light-box, the burners must be placed in it properly.
If no light-box is available, a clear window may be utilized in "blocking" the negatives.
If wood is used, the light-box is apt to split under the influence of the heat within.