Solar Hot Water Cylinders often have 2 or more pockets where the temperature sensor is fitted.

These pockets are made from 10mm copper pipework and after a few years may be susceptible to leaking due to corrosion.

A 10mm compression cap can be fitted. An example of one from a Google search is here:

Often the end of the pipe has been damaged so the pipe will need to be trimmed carefully. This can be completed by a plumber or skilled competent person. In the past "dremmel type" multitools have been used.

If the solar system is still in use, then the sensor will have to be placed in a position of the cylinder wall in order for the solar controller to get an accurate reading of the cylinder temperature. This can be completed by a solar engineer or competent person.