Right now, for use with a tarp, I just use a 4'x7' piece of 2 mil(?) plastic dropcloth. I don't anchor it. I sloppily fold it, and then stuff it into a small stuffsack.
It works, but I'm planning to switch to a bivy (used with the tarp) with a silnyon bottom, and with an extra coating of thinned silicone caulk on the inside for increased waterproofing and reduced slipperiness.