Originally Posted by gargoyle
Differential cut is when the inner layer of fabric is smaller than the outer layer. It allows for a snug fit, yet the insulative loft is not compressed.
Dart is where you do a tapered (or small triangular) cut and sew to create a better fit.
Aside from those details...., I'd recommend adding a small drawcord channel to both the head and foot ends and cinching them up with some thin shock cord and cordlocks. Skip the darts.
The drawcord will seal it better.
But do something to seal up the ends.
Thanks for the info - I'll probably go with the drawcords.
My tailor does all my sewing for me. He hasn't yet master the English language so I have to physically show him what I want rather than describe it, but thanks for the details on the cuts anyway.