If the silk or microfiber liner does not bring the light weight bag up to acceptable, try these two, field-expedient tips that LITERALLY do not weigh anything extra, as you likely already have these items with you:
1. Wear your hiking outfit for tomorrow to bed, and/or;
2. Use your poncho as a layer to prevent heat loss. Lay the open poncho on the bunk or mat, lay the sleeping bag on top. After you get in the bag, "wrap" the side of the poncho over you, as though you were making Chinese egg or spring rolls. I find I get the best coverage, plus added vents, if I lay the poncho on the diagonal. The "ends" are easier to wrap, and the opposite diagonal provides better coverage. A square poncho is longer on the diagonal corners...
Finally, wear a hat or Buff to bed. If you keep your head warm, you will radiate significantly less heat from your body, into the space you are in. If y0ur hands are outside the sleeping bag, wear gloves or an extra pair of clean outer socks as mittens.
I hope this helps.