I milled a little brass ring which is soldered to the copper tube. The ring fixes the lense.
Brass can be found in the plumbers junk box.
From my point of view, the diagrams from DD9FJ above show an effect I could see also. The increasing of the gain is less then the reduction of the background noise. The diagram gets smaller when using the lense and there is less noise receiving beside the dish. As a result the SNR is higher.
This can be seen more clearly, receiving the NB with a wide spectrum display.
So, the lense works as expected.