This is a fairly crude method however as the best modulation characteristic usually occurs at a specific voltage. You can mechanically tune the module by adjusting a tuning screw, in and out of the cavity. In this way they achieved an accurate, resettable tuning system. This photo shows some waveguide 16. It has a square coupling flange fixed to it so that it can be joined to more waveguide.
Round flanges are also common and can be linked to each other with special coupling rings.
The square flanges are joined together with nuts and bolts. It is also available in the USA and Canada. At the end of this article you will find a list of suppliers of this and similar modules. It usually comes complete with it own antenna, a small horn. The diagram following the photo shows the internal construction of the unit.