Aboveground Rovaseal Wide Mouth Winter Skimmer Plug (Min/Max 5-1/8-3/16 H x 11-1/2-5/8 W Inches) The Rovaseal Skimmer Plug snugly fits like a cork into skimmer face opening ensuring a water-tight seal. Measurements: (Min/Max 5-1/8-3/16 H x 11-1/2-5/8 W Inches) With no installation or tools required, the Skimmer Plug securely fits into the skimmer opening using the exclusive Rovaseal thermo insulated layer to plug off and grip the faceplate, creating a 100% air-tight seal. The Skimmer Plug uses a proprietary foam that blocks water for use in aquatic environments. Rovaseal is a high density foam. Characterized by a compact feel and resistance to water, RovaSeal has many of the same properties as neoprene rubber.