Hello chaps,
first of all: Many thanks for your various replies. Very helpful!
I have Rimowa aluminium suitcases in different sizes so I think the idea with the Pacsafe net could be the best one, perhaps in combination with a security cable. Must check the different sizes of my cases and choose the fitting net size.
I will fit my luggage rack for the first time next spring/summer for a trip from Germany to Malvern including the ferry and perhaps some stopovers. We´ll take the big case (two persons, 4 days) to avoid to have to much stuff inside the car.
When I went to Malvern last year we also took the ferry. I was wondering if the cargo bay of the ferry would be camera monitored during the cruise. People have to leave the cargo bay but what If somebody would hide e.g. in the trunk compartment of his van and then takes his time during the cruise to "check" the cars in the cargo bay unhurriedly... I´m really not a pessimist but I´m not interested in experiencing any unpleasant surprise after returing to the car...
Thank you all and have always a safe trip!

Kind regards,
George
P.S.: Pandy, perfect solution! Really looks very scary...