Should the students be allowed to place a separate "plug" or "cap" into or on the opening of the internal 250 mL beaker before closing the device?

Yes, teams may put a "plug" or "cap" on the opening of the internal beaker. Anything put in or around the beaker cannot physically modify the beaker nor the water in the beaker. The required hole for the probe must be at least 1.5 cm in diameter in any plug or cap. The device does not need to be all one physically attached unit.