I really like your simply bivy design. No zippers, mesh, etc.
What about making a similar bivy using basic 1.1 oz dwr ripstop and 1.3 oz silnylon? The bivy would not have a WPB top if you replace the Propore with DWR ripstop but most bivy's around here are not WPB anyway.
You could buy 2 yards of ripstop 2nds and 2 yards of silnylon 2nds for about $24 + shipping. You could cut the fabric into two 36x60 rectangles to get the desired length. With seem allowances that would give you a 70 inch girth which should be enough for small to average sized folks. The only downside would be that you would have to sew an extra seem across the center of the bivy.
When you consider that the polycro and propore will be coming from separate vendors and that the silnylon and ripstop can be purchased together from OWF, Quest, or thru-hiker these two projects would have about the same cost after you factor in shipping.
I know the ripstop and silnylon could create a 6 oz +/- bivy.
So, I guess I don't really understand why the Polycro/Propore bivy would be any more SUL or SUC than the standard dwr ripstop/silnylon bivy.
If the ripstop/silnylon bivy is just as light, just as cheap, easier to sew (you mentioned the polycro was difficult to sew) and probably more durable, what is the advantage of the polycro/propore bivy?