I have always been pleased with the Snugpak line of products. They are carrying a poncho now that is pretty good.
Way oversized in the back to cover your pack. Short arm sleeves to cover them as well.
Weight- 13 oz / 370 grams.
Found mine on Amazon for about $48 bucks.
Another piece of gear for my son and I to fight over who gets to use!
Here is another good picture showing all the features.