An adaptor includes a cap and seal and may be easily attached to a container after contents have ben placed in the container. The adaptor includes a seal on its bottom surface which is pinned between that bottom and an upper edge of the sidewall of the container after assembly. This provides a seal which cannot be replaced after opening the container. A second seal covers the first seal to provide additional air-tightness during shipping. The adaptor is securely attached to the container, and those locations at which one could pry the adaptor from the container are provided with tamper-evident features.