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!

[u/WhilstTakingADump]

Can I leave the bag zipped closed, or should I plan for it to be open? Open would need different placement for the webbing.

You are going to need the unzipped width IMO for this style of attachment. You eat up a lot of the width when you sleep on the diagonal, and while there’s a bit you could do to also make the UQ also more diagonally aligned with you it’s going to take some tinkering. 

Is gathering each end with a bungee a good approach, or is there a better suspension method?

I believe that people have the best luck with multiple bungees, the more the better though but with such a strong pull you start to run the risk of the UQ being pulled too tight to the bottom of you. You want it snugged up with no airgap, but not at the expense of crushing your insulation. It’s a fine line so I would build in plenty of extra bungee length to play with. Unfortunately if the insulation in the bag you are trying to repurpose is too heavy you may never be able to get it where you need it with this stretch but not too much stretch you need.

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?

Outside, yes. Inside the gains would be minimal and IMO not worth it. 

Anything you’d do differently or that I haven’t thought of?

20f is very cold, and from what I see of that insulation (hard to tell the loft) I don’t think that’s going to cut it. It looks dangerously thin. I’d take a look at the insulation thickness of an off the shelf UQ for that range to get a sense of what you will need loft wise. You’d definitely need to ease into your temps in a safe way to see where your temp range is. Remember, your top quilt/bag you are using won’t offer much help for the bottom side, because that insulation is getting crushed. 

Be safe! Seems like a fun, albeit challenging project! Above all, try to build with flexibility in mind, because once you hang with it you may need to make some adjustments. GL with the build. 👍