Carton Holder — Get a grip

The thing I like about this iteration as opposed to others I've seen is the truncated height and the relatively minimal use of plastic, enabling you to more easily visualize the contents.

From the website:

Carton Holder

Pouring from square cartons is easy with these lightweight but sturdy plastic holders in two sizes.

Simply slide in milk or juice carton to add an easy-grip handle.

Fits most refrigerator doors.

Dishwasher safe.

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Quart-size $4.99, Half-gallon $7.99, both here.