I was going to post the same thing! It's such an easy idea, I think I could almost come up with my own pattern! But I'm not that confident.
Hopefully someone will chime in!
Just found this with a quick google search of "free soaker sack" : http://madquilter.blogspot.com/2007/...l-pattern.html
Obviously I haven't tried it though!
Just had another idea- it's not knitting though. You know how you can make longies with sweater sleeves from thrift store sweaters? What if you use the body part of the sweater, neck for waist- take out arms and sew them up and and turn it into a sack? You'd have to find a nice small sweater though.