One technique I use for a quick render is to create a luminous dome (polygons facing inwards) and a simple square for the ground. Niether of them need to be anything special, but so long as the dome is luminous and effects the radiosity/lighting and the ground doesn't it should work. I also recommend making both invisible to the camera and to have a point light to add a bit of definition. The trick is to balance the light from the dome and the point light.
...That works well and quickly in Lightwave anyway!
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