Underquilt from an old sleeping bag: advice wanted

[u/ATurtleStampede]

I want to make an underquilt for late-fall car camping in my hammock, mostly in the Colorado mountains (I’m guessing lows in the 20s°F), and I’d like to keep using it in Virginia after I move in December. I’m repurposing an older, heavy synthetic Eddie Bauer rectangular bag (35 in. wide zipped, 70 in. when opened flat) plus its thinner liner (35 in. wide when opened flat). I’ll sleep in my REI Igneo 17 on top.

My plan:

• Sew eight 8 in. × 2 in. webbing loops with a box-X stitch, four at each end.
• If I open the bag flat, sew the liner down the center for a double layer under my body.
• Cut the ring off a six-legged bungee “spider,” tie two of the cords into loops (one per end) through the four loops, and clip them to the tree strap with a carabiner.

Questions:

• Can I leave the bag zipped closed, or should I plan for it to be open? Open would need different placement for the webbing.
• Is gathering each end with a bungee a good approach, or is there a better suspension method?
• Would a mylar layer on the outside as a windbreak be worth it, or would sliding it inside (between the liner and bag if left open) add warmth?
• Anything you’d do differently or that I haven’t thought of?

Thanks in advance for any advice!