Has anyone tried two pieces of tyvek house wrap? You could seal it across one side and the bottom, use velcro for the other side.

Limited, but good.

I bought a Sierra Designs Zigori in 2006 because the price was right and I wanted a bivy sack. I've spent two nights on hills/mountains exposed in Idaho in the summer. On both occasions the mesh kept me bug bite free and it rained in the morning and I zipped it mostly closed and I stayed dry. Would I want to camp in a week in it? No.

I have also slept in duck blinds where there's mostly a roof with it. Worked great.

I've slept in it various other times when it was say, super wet, and I was in a tent, or on a balcony in Germany, etc.

I never regretted sleeping in it, but know its limits and best case uses.

I'm using a US army goretex one.