From various responses, I suggest that you get yourself a lightweight pack cover. If it fits your budget, consider getting this in cuben fabric then --- very light, and repairable with duct tape:
http://www.zpacks.com/accessories/pack_cover.shtml

I used this model (and their cuben rain skirt too) on the CDT a couple of years ago, and it held up well to occasional use over that 4+ month period. Got one little hole poked in it and indeed, easy to repair with duct tape.

If the "lean it against a tree at night" solution doesn't work for you, you can carry that pack cover AND a 1 ounce yard waste bag and still come in way, way lighter than a 370 gram mostrosity --- IMO. Not as durable, not suitable for using as checked baggage, but if you have that as a problem, I'd solve it separately, in a way that keeps your on-trail weight low.

Lots of approaches to this stuff, this is just my best guess for you based on what you've been saying ...