An apparatus includes a main body substantially enclosing an interior volume. The main body is elastically expandable to an inflated configuration. The apparatus further includes a vent aperture in the main body and a vent cover at least partially overlapping the vent aperture. The vent aperture and the vent cover define a passage from the interior volume to outside of the main body. In the inflated configuration of the main body and within a pressure threshold of the interior volume, the vent cover constricts the passage proportionately to a pressure increase in the interior volume of the main body.