Thanks but I don't think Cunard would let me bring the hire car on board Queen Elizabeth
Erm what ? Ever heard of one-way car hire ?
Having just Googled "Civatavechia car hire" it seems like all the main brands (Hertz, Europcar,Sixt etc.) have offices in Civatavechia.
So, just book pickup at FCO, drop off at Civatavechia .... and Robert is your uncle.