Differential cut:
For a given girth, say 45″, model that as a circumference and than calculate the corresponding radius. So, 45/2pi = 7.16. Now add you loft to that and calculate your outer girth. 5oz Apex has a loft (I think) of 1.2″, so: (7.16+1.2)*2pi = 52.53″.
Say you want a 5″ gap between the edges of the quilt underneath you. Add that to the girth of your inner layer and repeat the above. Then calculate the ratio between outer and inner layer girths (aka circumferences). It should be > 1. Multiply the gap size by this and subtract the result from the calculated outer girth.
When you sew the inner and outer layers together at the head and foot seams, you’ll have to fold/pleat/gather the outer layer a bit to get it’s length to match the inner layer’s length.
A simple approach to a draft collar is just add 3″ to your total length beyond the draw string at the neck. Works reasonably well for the effort to make it.
For the poncho slot, you need vertical walls inside the slot (like baffles in a down quilt) so you don’t compress the insulation, and you want a flap to seal the outer layer from drafts when you’re sleeping. Velcro will seal better than snaps, but it has a scratchy side you don’t want against your skin. Omni tape addresses this but I can’t speak from experience. Take a close look at how others have done this.
Good luck! Share pics and spec’s when it’s done!
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