Another way is to cut a piece of styrofoam (like you'd get in a cooler container for refrigerated supplies) to fit into your tupperware. Lay several sheets of paper towels over the foam, place your specimen on the paper and pin. The paper towels allow the formalin to wick between the foam and the tissue. You can float the specimen upside down in the formalin or use something heavy to weight it down.
It's cheap and when the foam gets nasty you can toss it and replace with a new piece.
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