Making a synthetic “blanket”

0.67 oz fabric from Titaniumgoat.com

or 0.9 oz from thru-hiker.com or owfinc.com if that's not available

Apex from owfinc.com or thru-hiker.com

or any of the primalofts if that's not available

the 2.5 or 3 oz weight might give you 20 degree F lower, some people might think less than that

Thanks for the feedback. I am not looking for the overquilt to add any warmth to the sleeping system, although it will for sure. I just want it for a "frost guard" to catch the condensation instead of my down sleeping bag.

You are describing essentially a poncho liner. They measure 62"-68" x 82". Here is an example. It isn't the lightest thing in the world, but it probably costs a lot less than the time and effort of sewing your own. Just a thought…

http://www.rangerjoes.com/Poncho-LinerWoobie-P1949.aspx

If you are set on MYOG, that's cool. If not, check out the military surplus poncho liner. A thin, very light, square quilt blanket. Just shell and a bit of synthetic fluff, nothing fancy. They are very roomy, I assume they could fit two bags. Definitely two people could sleep under one buy itself with plenty of room. They weight right around 1lb.
They make great underquilts for hammocks as well when it gets around freezing.

Edit: oops, marc beat me to it.

If you want the 'blanket' to catch the frost/condensation ratheer than your quilt/SB, then it HAS TO add insulation to the combination.

We (Sue and I) often use our light down summer quilts in winter with a winter down overquilt on top. Works very well. Shared warmth under it. Yes, it keeps the inner quilts dry.

Hum … it seems the overquilt dries off a bit in the late afternoon when we have pitched camp. At -40 C there could be more of a problem I guess.

Cheers