You might look at something like the MSR E-Wing or equivalent - they advertise them as being an emergency shelter. I don't know how pricey they might be. I think Integral Designs might also make some tarps along the same lines.

Personally, for what you're describing, I'd probably go with the lightest, cheapest thing I could find. I did a quick check, and found a $60 silnylon 6'x8' plain old rectangular tarp at Campmor. Coated nylon tarps are heavier, but cheaper, and widely available.