I now rig my rectangular tarp for winter use like bearpaw do
Picture #2 at http://www.hammockforums.net/forum/s...ead.php?t=6488
I set the ridgeline a bit higher & the "doors" will actually close completely. The bottoms of the door flaps are secured with elastic cord, so to open all I need do is unfasten the bottom of one flap from the tent peg & step out. I can if need be still fasten the door from the outside, but I'm usually not outside that long so lI often just eave it go.
Last time I was out, I wasn't exactly sure it helped till the NW* door flap came loose, I noticed an almost immediate drop in the (percieved) temp. As the outside temp was only down to 23, I didn't bother fixing the flap till hours later, (didn't seem worth the "effort") but I did notice.
*I was hanging oriented N / S, the wind was mostly from the W & NW with minimal natural wind break.