A device for retaining the blank or sealing lid of a split cap used to cap a fluid container having an invertible spout. The device includes various means for ensuring that the blank will be retained in close proximity to the user so that it will not be lost. This is achieved by modifying the blank either to enable affixing it directly to the container, or to configure it to be easily attached to a site close to the container. The retaining device includes a threaded section affixed to the blank for coupling the blank to the standard vent port of a container. Optionally, the retaining device includes a tethering hole located in the gripping flange of the blank to enable tethering of the blank to the container or to a shop hanger. Alternative designs of the retaining device include placing attachment devices on the underside of the blank and on the container, such as hook-and-loop couplings, male/female nipple sets, and magnetized components. The container may also be modified to include a pocket for stowing of the blank.