In reply to Stockholm Syndrome (USA): bivied in a mountain equipment moonwalker 4, with a crappy foam mat, a fleece liner and a cheap decathlon bivi bag... my face got a bit cold in the middle of the night but i'd built the snow up around me a bit so there was no breeze. Any technical synthetic bag rated to those sort of temps should do the trick i guess.
If its just for the experience then do it as cheap as you can
thats a bit of a ramble, hope some of it is useful?
C