two ways of doing this
1) GI_Joe - search for that on this website an ull find it - its a plugin
2)manually - make a pshere scale to bigger that ur scene
reverse normals so they face inwards
apply a lambert to it and chose a colour
wack incadense and ambient up high (tweak these by doing test renders)
leave lambert 1 on ur model or u can make a new lambert if it needs tweaking
render in mental ray with final gather on at about 500