i'm not sure how to best answer that question but i'll give a couple thoughts & we'll see what the others have to say.
i believe a lot of folks would suggest a tightly weaved fabric.
a DWR treatment would make it even more down proof i believe but in my limited experiment w/ home made down stuff, i'm thinking that an untreated fabric will do fine as long as it's tightly woven.
ed speer showed me a way to test for tightness in the weave. take a corner or edge of the fabric & grip tightly w/ both hands a couple inches apart & pull up & down in opposite directions, stretching the fabric.
with moderate force, it should not begin to show light between the threads.
but if you do it very hard, you can cause even the fabric that is sold as "down proof" to show seperation, so it's not a very scientific test
i've used 1.1 rip stop from the $1.00 bin & it seems to do fine. ...tim