Does such a product exist?

Have a look around at sites specializing in boating like kayaking.
They need larger dry bags because everything they are carrying might get dunked.

great idea thank you!

Other than purpose built drybags as has already been mentioned, you could also look for 6mil bags. A local asbestos abatement contractor would likely sell you some or you can get them here Pack Liner. They’re much more resilient than garbage bags and can easily be mended with duct tape. You could probably get a cuben fiber drybag custom made by one of the cottage manufactures who specialize in cuben fiber.

What about making your own bags? The advantage would be that the bags would perfectly fit your pack without a bunch of excess heavy plastic. I would think that a hot iron would work with some plastics, otherwise adhesive or tape.

For really heavy duty waterproof bags check out Seal Line Baja dry bags, I get them on Ebay and use them for kayaking. I think there about the toughest made. They make really large ones. They are not cheap, but there are occasional bargains on Ebay, sometimes on the Milspec versions Marines use.

I will look into these options and find out what works best. What a helpful community; I appreciate all your responses, thanks!

I would look at the pack liners at Zpacks. They are custom sized. I bet Joe can make a light cuben dry sack to suit your needs.

ULA epic

http://www.ula-equipment.com/epic.asp

Just use a rubble sack, it will be far more durable than a normal rubbish bag, but still incredibly cheap and light weight. You could probably take a couple of spares and still be saving weight over a dry bag.