Here's a 4.3oz rain cover for 80liter backpacks. It's called the "Duck's Back" by REI. I think I used it once...pretty much excellent/new condition. Comes with a carrying case that can attach to backpack compression straps or daisy chains. The rain cover is adjustable with a cinch drawstring system. You could shave the weight down more if you replace the drawstring and cordlock with a lighter one.
Here's a link to the current and heavier version: http://www.rei.com/product/778477/rei-ducks-back-rain-cover-80-liters
$14 shipped. Paypal only. Personal/Gift payment preferred, otherwise add $1 extra for other paypal payments
Mine looks like this: