I have a trek speed concept gen 2 with bontrager hilo pro carbon bike saddle, i bought a gen 3 and i tried to install the carbon seat but it was not possible because on the carbon sea the carbon bar and thicker the aluminium bar equipped on the original seat

On my gen 2 i had the carbon seat installed with the bontrager carbon seatpost of the picture enclosed, but apparently it can not be installed on gen 3 because is diameter is a little bit shorter than the original one

So in summary, how can i install the carbon seat of my gen 2 in my new gen 3

Thank you very much in advance for your support in this matter