I presume you mean the 17L bag that goes inside the Saddle Holster? The problem with these dry bags bags is the fact that they are shaped to fit into the holster and this is part of what makes them stable and less prone to swaying.
One possible alternative might be the drybag from the Topeak Backloader bag, which has an air valve. You should be able to buy them separately and they have a taper too them as well which might fit into the Restrap Holster without too much bother.