Made a new bivy using Tyvek home wrap, nylon mesh, and velcro yesterday. Took a piece of tyvek, cut it to size and dimensions (feet narrower etc), folded it up on the feet end to about where my shins are, taped it closed. Then sewed some nylon mesh to that end and to one side, then sewed some velcro to the other side of the tyvek and mesh. Sewed a loop on the mesh for a tie out point. Weight of the bivy is 11.4 oz. It's more of a bug bivy all in all. As mentioned it cost about 10 dollars in materials. Going to try it out this weekend at Shenandoah N.P.
I plan to use it with my Sea to Summit Ultra Sil poncho-tarp when expecting rain. Weight is i think 12 oz, plus weight of cord and AL stakes.
In a little while, i'm upgrading to a Zpacks Cuben Poncho/Tarp that i've requested and Joe agreed to do for a reasonable price. I will combine that with a Sea To Summit NANO pyramid Mosquito Net, which i will sew some cuben and velcro onto.
However, that combo is going into my ultra durable, but lighter, save for later stash. Course i will have to test it out it a few times.