Go ahead and get a ptarmigan bivy and save yourself the learning curve. A tarp is great, but mist and splatter will find their way to you and your bag. Of course you could make your own bivy if need be. Mine weighs 12 ounces - it has a pu coated bottom, a dwr top/ the upper 1/3 of the top is noseeum with a zipper from one side to the other about chest high. In the summer I usually fall asleep on top of my sleeping quilt and then in the wee hours of the morning when it starts to get cool I end up in my quilt. I have a tyvek bottom, full dwr ripstop top bivy that I use in winter. Its purpose is to keep my underside dry and to add warmth to my sleep system. In summer I use an 8x10 Equinox tarp (16 oz). In winter I switch to a Sierra Designs Oragami (28 oz)- It provides more shelter than a tarp and can handle snow loads very well, and after 33 years of backpacking I must say that it is the most versatile shelter I have ever owned.