A little while ago people were talking of using tyvec for various things and I managed to get hold of some from MM-on-point (thanks again) from the classifieds on here.
I wanted something big enough to take my new Exped Lite down mat, let me roll over without moving the mat and be able to wear my down jacket to sleep, this is the result which weighs in at 225g.
Bivi bag with mat and sleeping bag inside.

Tyvec Bivi Bag 001 by youngray50, on Flickr
Envelope style hood (will always be used under a tarp so not worried about being able to close the opening up tight).

Tyvec Bivi Bag 002 by youngray50, on Flickr
Envelope hood closed.

Tyvec Bivi Bag 003 by youngray50, on Flickr
Seams where made using strong double sided carpet tape and are 2" wide. I was going to sew them as well but found the sticky tape kept gumming up the needle and snapping the thread on the machine, I think the seams will be strong enough as they are.