To benefit form a GI, you need the garbage bags. A crumpled space blanket doesn't do much just tossed into the poncho as you discovered.
The next test should be the following: Rig the poncho like before. Take 2 large garbage bags, the largest volume you can find. Open the bags and allow just enough air in, so that when you tie them closed, they are only a few inches lofted. Put the crumpled space blanket in one or them. Place the bags in the poncho with the space blanket one at your torso and the other towards your legs (or use two with SB's). test again.