Did you ever look into the feed sack material?
It's pretty much exactly what you're trying to make, ie, it's a fiber reinforced plastic material.
It's lightweight, waterproof, durable, and you can sew it. Plus, it's proven to carry a 50lb load and if you have any friends with big dogs or horses you've got a source to get plenty of it for free.
Check it out next time you're at the grocery store, it's pretty impressive stuff for your intended use. It may not be lighter than silnylon, but it's easy to get and a whole lot cheaper!
I've got a coworker with both dogs and horses, so I may be able to get a hold of some pretty easily.