Suitable caps for this item would require 8-425 threading. There are several 2 mL vial cap options in various formats available.
Please see the link below to review potential options:
https://www.sigmaaldrich.com/search/cap-8-425-2-ml?focus=products&page=1&perpage=30&sort=relevance&term=cap%208-425%202%20ml&type=product
To discuss this further, we kindly ask you to navigate to the link https://www.sigmaaldrich.com/techservice and click on "Product Technical Inquires" under the Products Section with all the required information so that a member of our team can reach out to you to assist further. Thank you.