I don't know if you do any sewing, but if you do (or someone you can wheedle does :) ) there is the Parcho, as seen here:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=61864
There is a free pattern on the web, but I chose to purchase one from Quest. I made it of silnylon, and made it longer than the pattern, just below my knees (for a total weight of just over 7 oz, sealed); so far it has worked out well, though the past 2 years have been drought years so it hasn't had much use while backpacking. If I were to do it again I would leave the seams open under the arms for added ventilation, or maybe just put snaps down the side rather than sewing the seam, so that it could be used in a more tarp-like fashion in a pinch. I added grosgrain with snaps at the shoulder seams, a la Roger Caffin's Mountain Poncho, so it will stay attached to pack even if I'm not wearing the Parcho:
http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_mntn_poncho.html#.VaPwB_kUO_Q
If you don't get prolonged rain, you might leave off the hand flaps on the Parcho; that would add more ventilation.