These Zipp integrated valve extenders are the preferred valve extender because they allow the valve to be closed unlike the fill tube type that is installed over an open valve. These extenders only work with tubes and tubular tires that feature a removable valve core. Simply remove the valve core with a valve core tool or small adjustable wrench and re-install into the end of the extender.
These extenders are nicely machined and made of lightweight aluminum. Zipp Integrated valve extenders feature an integrated air seal. One extender in each package so order two extenders if you have two wheels. For sizing on a non-Zipp wheel, a good rule is to have the total length of the extender plus the length of the valve be at least 20mm more than the rim profile.