I use dry sacks with my pack, strapped on with 1/2" polypro straps.
My first dry sack, a Sea to Summit 'lightweight dry sack', is soft and flexible. The surface of the sack is not too slippery, so it is not necessary to get crazy with cinching down the straps on the bag.
I decided that the 20l bag was too big - although it could hold all my critical dry stuff, it required longer than usual straps and stuck out a mile on either side of my pack.
So I replaced it with an 8 liter and a 13 liter bag. By the time I went to REI to get the bags, the original S2S "lightweight dry sack" had been replaced by some labeled "ultra-sil".
These bags are less flexible and more slippery then my original bag.
I've used these replacement bags for a while, being reasonably careful to not abuse the bags.
When packing for my most recent trip, one of the bags ripped - a small slit parallel to the opening of the bag. It was close enough to the opening that I could rearrange my stuff and roll the closure down past the slit.
Looking at the location of the slit, I think it could have been caused by my cinching down so hard on the slipperier ultra-sil bags. The straps could have caused abrasion, leading to the tear.
Checking at a REI store, yielded only the ultra-sil bags and some newer and even more expensive S2S eVac bags. Online, REI still offers the "lightweight dry sacks".
I would recommend the more inexpensive Sea to Summit bags for use with the Molly Mac Pack.
Here's me and my pack.