A fill cap for a drink container for receiving an open end of a drink bottle. The fill cap includes a cylindrical collar and an annular flange extending radially inwardly from the cylindrical collar. A valve structure is connected to and surrounded by the annular flange and in includes a pair of generally planar lip members. Each lip member has an outer portion adjoining the inner edge of the flange and the lip members converge at an angle axially downwardly to inner edges where the lip members adjoin each other at a slit. A pair of opposing side walls extend from the flange to edges of the lip members located axially downwardly from the flange. The slit is configured to receive the bottle neck therethrough.