Sweet tip Rick, thank you! I saw your post and made the conversion with my crappy Neo air pump sack the same night you posted. Took off the valve and put it onto a plastic lawn/leaf bag, and used some tyvek tape to reinforce.
Worked like a charm this weekend. Took about 3 to 4 cycles to completely fill up the Neo Air all season pads we used (each, not total).
Not sure how durable it will be--would like a more durable version as i dislike throw away materials on principle. Have a bunch of silnylon left which i'm not using, might make a long lasting pump sack out of it. Actually, have a couple of large homemade stuff sacks made out of it, and if i seam seal it, should work well.