A clamp for an Erlenmeyer flask or other laboratory containers or racks uses nickel-coated, rare earth magnets to secure the clamp to a platform for a laboratory shaker. The base of the clamp has downwardly extending positioning bosses that seat in holes or indentations on the shaker platform to prevent horizontal sliding of the clamp when the shaker is in use. A removable and replaceable, elastomeric cover for the base of the flask clamp provides cushioning and prevents spinning of the flask when the shaker is in use.