The sturdy deep green base is grooved to create a reservoir for water so that the planting block floats. The clear fitted top includes ventilation slots you can keep closed or open to desired size. Very durable and attractive, this unit fits all size planting blocks (the 60-cell, 40-cell, and 18-cell) and is the perfect size for placement on a heat mat. The base measures 14 7/8" long x 9 1/2" wide x 2 1/4" high, while the clear top measures 14 7/8" long x 9 1/2" wide x 4 1/2" high.