I use a nylon backpackers pancho I purchased at a local sporting good store. Its long enough to cover my pack or clip up when its just for me. I also wear my wind shirt which is water resistant underneath if its windy and raining. Works pretty well. Campmor sells a sil nylon verion thats 20$ more but 1/2 a lb lighter. seems like a good price for a sil nylon with webbing sewn in. The sea to summit sil nylon pancho is longer so would cover more but almost double the price.
silnylon pancho
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My gear is no where near lightweight