you can make a similar (and far lighter) bivy sack by ducttaping two space blankets together... on one side only tape 3/4 of the way up though or its a REAL pain to get in/out. Add a lightweight fleece blanket/thermal undies and I've been comfy down to near freezing with it.
**Also, If you set the bivy bag properly when it's raining, you don't really need a tarp. You can fold the top edge of the bag and put an old curved tent pole in it to keep it up off you. If you really want you can make another "channel" for a straight one on the top of the bag going lengthwise to keep it up off of you while you sleep.