Garbage bag with small hole in bottom corner, held onto valve with rubber band. Takes only 2-3 bags full for my Kooka Bay mummy style (3.5" thick, but I don't blow it up that far; I keep it really squishy). I also use the garbage bag to cover my dog's pack at night (multiple use, of course).
The Exped inflator bag sold with the UL7Downmat (or sold separately) sounds like a good idea, especially if also used as a pack liner. I tried the BA Pumphouse but it is (1) too small--takes half a dozen or more fills to get enough air in, even with a squishy pad, (2) not suitable as a dry bag for sleeping bag storage because water leaks right through the fabric, (3) a failure as an inflatable pillow (flat in about 15 minutes).