I have an upcoming Colorado thru-hike that I'm planning gear for, and one of the issues is whether or not to get a bivy for my quilt for use under a poncho tarp. So the ideas here are specific to this purpose.
In my case, the primary functions of the bivy would be:
2) draft protection for windy nights
3) protection from blown rain under a somewhat scant (5'4'' wide) poncho tarp
4) (possibly) bug protection
What is NOT necessarily needed is additional warmth (I will have a really warm down quilt)
I've been looking at commercial bivy options and nothing really seems to be a perfectly optimal solution, especially considering the cost ($140 and up) for a bivy to my specifications. They generally make it harder to get up to pee in the night and limit your ventilation options.
So here's what I'm thinking of doing. Cut an double Adventure Medical emergency blanket to size and duct-tape the corners to create a bathtub style floor that would be high enough to block ground-level drafts from creeping under the quilt. Then equip the corners -- if necessary (I'll have to see) -- with tie-outs that can be tied to the poncho tarp above, probably via elastic cord. I think this would adequately perform the first 3 functions, and for the 4th (bug protection) I would wear a headnet of some kind to sleep when necessary (often not necessary after dusk, as I understand). This would give me the freedom to sit up, ventilate as necessary, get up at night, and generally not be at all constricted by the bivy. And it would weigh under 4 oz total including the headnet.
Has anyone tried anything similar? Are their problems with this idea that I have overlooked?