I recently had Joe at ZPacks make me a 18x36 dry sack pack liner out of 1.26 oz/sqyd cuben fiber. Since Joe sews his cuben I had to seam seal it, but before seam sealing, it weighed 1.7 oz and after sealing it it now weighs 1.9 oz. Plus it is an envelope shape which means it tapers around the edges rather than rounding out. This design, and size, fits perfectly inside my ULA Circuit which also comes to a taper at the bottom. So, the shape does not alter the shape of the pack at all, and it fits inside like a hand in a glove. Sure, at $35 it costs a little more than a trash compactor bag, but it will last much longer than any trash compactor bag, and IMO it will be more reliable in the field (meaning I am way less likely to accidentally poke a hole in it). Check out my video review of it here on my blog:
So, everything inside the pack is safe, but I still keep a few other items inside cuben or sil stuff sacks for extra protection. My bag/quilt is inside a large MYOG sil stuff sack at the bottom and my clothes are in cuben stuff sacks. Then other things like my ditty bag, and kitchen set-up are also in appropriate cuben stuff sacks.