I feel dumb. I’ve been looking for a scooper that Mr. Savvy can use for scooping grain when brewing beer, when all this time I could have just used the top of a large bottle. Real Simple suggests a bleach bottle, but a one-gallon milk jug would also work.
In addition to these four ways to store plastic bags, you can also use a coffee can to store plastic bags.
Finally, apparently you can use the top of a water bottle to seal a plastic bag. Cut the top off the bottle, fold the plastic over it, and screw on the lid for an air-tight seal and easy pouring. (I’m not sure it works with thicker plastic, however.)