The photos below are self explanatory. They give you an idea of how to construct quickly and easily a tunable element to be installed n various types of antennas, such as dipoles, yagis, log-periodic, verticals, etc.
On these photos, the 2 different tube diameters were considered: One with 12mm and the other with 14mm. Of course the choice depends on the application and priorities such as durability, etc.
The tubes are aluminium and the collar is conductive (metal). You might get rid of it if the outer tube fixes well with the inner tube on the ‘sliced’ point.