What I would consider making if I could make my own bivy,
It would double as a full protection bivy and a summer bug bivy.
A waterproof bathtub bottom like others, but the top would be two layers, the bottom layer would be full no-see-um mesh and the top layer DWR ripstop material like others use.
The top layer would have a zipper that would extend beyond the full length so that it could be zipped down and rolled out of the way when only the no-see-um is wanted in hot buggy weather, then zipped up as far as needed when the wind is blowing rain in under your shelter or during light rain while cowboy camping.
This of course would weigh a few ounces more than your typical UL bivy, but would be more practical for trips that could hit extremes of hot buggy to cold and wet. A thru-hike comes to mind.
The flexibility could also reduce the size of tarp needed.


