just got a roll of tyvek n' cut to fit
seriously, elaborate bivy bags are retarded - ultra-minamilist is best - i have some sort of basic marmot thing - it's water-proof no matter what and if it's pouring out, the best bet no matter what you have, if yer so fawked as to only have a bivy, is simply to get pissed n' pass out.
simple bivy bags are nice b/c you can carry them along in winter in addition to a tent, especially if you have an also recommended light sleeping bag, given the moderate nature of our ragged-ass hillz