I am interested as well in making a bivy sack out of these materials. Any thoughts on pattern cutting? I've sewed various "everyday" fabrics before, using patterns and not, but haven't yet cut my teeth on silnylon or epic or pertex or any of those. I have some thru-hiker.com stuff sack kits that I will make on day, but I like the idea of having a bivy and a small tarp for less than $200/each! To make the pattern, would I just trace the outline of my sleeping bag and add an inch or two? Where do you recommend placing the zipper?