Last year, I brought a Snugpak "traveller" sleeping bag with me. It was inexpensive (if bought in the UK). The poly-filled bag weighted about 900 grams in it's stuff sack - the size of a pineapple. Down is superior, but I am allergic to feathers.
This year, I cobbled together a solution that I personally think is brilliant - just IMHO. Though the forum, I discovered a silk sleeping bag liner available on e-bay from a company in Vietnam. The liner cost USD 12, plus about USD 9.00 to ship worldwide. I ordered the LARGE, rectangular sized liner as I am a stout fellow. The liner arrived in about 10 days and is an excellent value for the cost. Search the forum for "sleeping bag, Vietnam." You should find it. I think the vendor is named "Spirits of Vietnam."
To that, I added a tourist-intended, rectangular Cool-Max liner / sleep pouch I got on sale at
www.RickSteves.com.
The Cool-Max liner fits inside the silk liner. They are nearly exactly the same size. The Cool-Max liner is stretchy enough to make this work.
I attached the two liners together (nested them) using a ping-pong ball in each of the four corners with a hair twisty tie ("donated" by my wife). If you put the ball into the corner, inside both bags, you can apply the twisty tie to the outside and keep the bags together when you are getting in and out. It does not add to the weight. It adds to the compacted size a tad, but the big benefit is that the Cool-Max liner nested inside the silk liner is a damn warm sleeping arrangement.
As both the silk liner and Cool-Max liner have a "pillow pouch" at the top end, I had four corners to insert the ping-pong balls. I experimented with smaller spheres of varying sizes but only the ping-pong ball and solid 1.5 inch diameter styrofoam balls I got from my local Michael's craft store worked well.
NOTE: I originally tried this with safety pins, as the pins are lighter and could do double duty with laundry or emergency repairs. However, the Cool-Max and silk material is so fine that the safety pins tore through.
The 1.5 inch dia. styrofoam balls weight 1 gm each. The ping pong balls weigh 2 gm each. The Cool-Max liner weighs 242 grams and the silk liner weighs 187 grams. So, all in, this layered, field-expedient "sleeping bag" weighs about 440 grams "all-in. "That is about 15 pounces. Add a stuff sack for the assembly and we are looking at an even pound or less than .5 Kg. That is also less than half the weight of the Snugpack sleeping bag.
The bags can be used nested together or separately.
I hope this helps.