Thank you for your ideas, Jim.
Here's my new poncho. I still need to put fasteners under the arms. It's very basic and weighs in at 5.75 ounces. Here's the gist of what I did.
I asked my wife to measure from my adam's apple to knee, 100cm. Then from my first cervical bone to the knee while wearing my backpack, 140cm. I then measured out my silnylon with 100cm in front, 140 in back and the width of my neck divided by two as the center point and marked it.
After cutting the full length of silnylon to about 250cm, I made a + sign with a sharpie in the middleneck point (100(front) + 5(neck) ---- 140(back) + 5(neck)). The east/west axis was 18cm wide and the N/S axis was 24. This was determined by my wife holding a flexible tape measure to my head and guessing, nose to back of head and ear to ear. I then cut the lines of the + sign.
My next step was to double sew the diamond shape in the middle of the poncho, my head hole. I held down the folded back triangles with scotch tape and then sewed around the diamond twice.
Here's some pictures.
I then inserted some shock cord into the sleeves the triangle sewing made and attached a cord lock.
Here it is cinched:
Here's the full poncho:
And the weight, 162 grams, 5.75 ounces.
As I mentioned, I need some snaps under the arms to keep it closed on the side but otherwise, looks like it'll work for me!