I recently had Joe (from ZPacks) make me a cuben fiber dry sack pack liner. Envelope shape, 18x36 (about 39 L) and due to the taper of the envelope shape, it fits beautifully inside my Circuit. And since Joe sews his cuben I had to seam sealed it. 1.7 oz before, 1.8 oz after. However, I then added a little more at the corners and it tipped the scales at 1.9 oz.
I did a review and a video and posted it on my blog. Check it out here:
I had to contact Joe to get a price for it and use the custom button to order it. He can make them in whatever weight cuben you prefer as long as he has it in stock. I had mine made from the 1.26 oz/sqyd black cuben.