I can second the use of Sol Bivvies. Mine only weighs 109 grams. It has a zipper and a nice cloth-like lining on the inside that feels like cotton. The material has some thickness to it. This is definitely NOT one of the aluminum foil space blankets!

I also have a fleece blanket I got at Dick's Sporting Goods. It weighs 511 grams - so just over a pound. It too has a zipper and unrolls like a sleeping bag. There are two elastic bands at the bottom to keep it rolled up.

I slept in the fleece with temperatures into the 60s. I carry the Sol Bivvy in case they go lower. I'd think they two piece system could accomodate you down to the low 50s for 620 grams - 1.36 pounds.

The bivvy is about the size of a pop can. The fleece is about the size of a 1 liter soda bottle.

The fleece sleeping bag here: http://www.dickssportinggoods.com/product/index.jsp?productId=1419296