So I think I want to make a half bag/elephant's foot. Before I get materials and put real designs on paper, I have a few questions.
The inspiration for the project is the MLD 2Thirds Quilt and the Nunatak Arc AT. The quilt will be used with a Montbell UL Down parka or a UL Thermawrap - other torso clothes include Patagonia 1 LS and GoLite Ether.
My idea is to have the whole thing look like a cone that has the tip cut off. Like the MLD quilts, the footbox would be able to open and close with a draw cord. I could also sew the footbox flat, maybe. The insulation will be ThruHiker 800 down and with 0.75 inch baffles; baffle height is taken from Nunatak's Arc AT and is otherwise arbitrary. I would like the quilt to be good to approximately 35-40 degrees.
Construction would be to make the quilt flat, then sew the side edges of the quilt together to make the cone.
The first question pertains to the height. How far up my body should it go? The MLD Spirit Short (formerly 2Thirds) is 54" long and has a draw cord closure on the footbox. My gut tells me that it should go only my true waist, so that when the draw cord is tightened it cannot slid off me because of my hips. Accounting for a draw-cord footbox (5" radius when closed; my feet are 10" long), that is a 48" quilt height.
About that footbox. I have used a sewn-flat footbox (a la Jardine and the Bill's recent XP/Cuben quilt) and do not like it. I can sew in a footbox, 12" in diameter with one baffle, but is it worth it? I like the idea of venting. Will it be drafty if I do not sew it up or make a closure flap?
Second, what about going with a top-bag style and leaving no insulation in certain areas, such as underneath my butt and running down until the bag would start to fold over my foot. In otherwords, only the bottom three baffles would go all the way around; the rest there would only be a 10-12 strip of single-layer Momentum underneath me. Any thoughts? I am leaning away from this option.
Third, what about top and bottom widths? I want the bag to fit snuggly for maximum thermal efficiency. How many inches above my butt girth should it be? I am thinking approximately 3-6 inches. What about that bottom width? This is complicated some because if it is not wide enough, my feet will pull more fabric from the sides. The MLD quilt has a 32" bottom. Any thoughts?
Finally, how much down should I shove in there? With a 48 x 43 x 32 (Height x top x bottom) dimensions, that is 1800 square inches of insulation I need to fill. Filling to 3/4 inch would mean I would only need 1350 cubic inches of down, or 1.6875 ounces. That does not seem right - Nunatak uses 3.5 ounces in even their smallest Arc AT. Thru-Hiker only sells down in packs of 3 oz. In an e-mail conversation with Tom at Nunatak, he said a 1 oz overfill would be fine on an Arc AT, bringing the total fill weight to 5 oz and thus 4000 cubic inches of down. If I did that, I would be stuffing my baffles to approximately 2 inch height!
What to do?
Total weight? Sub 8 oz is the goal. My calculations show that the shell and lining will weigh 1.29 ounces each; that leaves 5.42 ounces for down, baffles, draw cords, cord locks and thread. Cost is about $122.14 from thru-hiker, assuming I buy 6 oz of down - about $121 cheaper than from Nunatak (assuming I do not go with 0.8 Quantum and get 1 oz overfill).