You will need: Plastic croquet canvas from micheals, 50 cents to a dollar
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Plastic lunch container from a dollar store
A lighter
Cut out the plastic canvas so that it touches the top and bottom of your lunch container, hold it in and with the lighter melt the top of the canvas thats touching the opening of the container. Hold it for awhile to make sure its completely fused together, and voila!
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Let it sit for awhile after you melt them together, I'm a little concerned about the melting plastic fumes entering the tank