When we moved here the removal company brought all our worldly goods in a massive removal lorry but we (my husband, daughter,then age 10 and i drove here from Cumbria in the north west of England to Gers in the south west of France, in our car with our Golden Retriever, our Guinea pig and our cat. We brought the guinea pig in a cat carrier and bought a bigger travelling case for the cat which more or less filled the boot, we put a litter tray in one side and his bed on the other side. Within about 10 minutes of setting off the cat had managed to open the zip and get out of the carrier and jump around the car. We put him back in and improvised joining the zips together so he couldn't get out, to say he howled was an understatement!!
We'd bought the carrier a month or so before hand and had it in the lounge with his toys in it and he played in it loads, just not in the car! Total waste of money.
So we got him out, put his harness & lead on him (just in case) and he spent the 2 days of the journey sprawled out contentedly on the back seat between our daughter and the dog. He wouldn't use his litter tray when we stopped for breaks all day (i was too nervous to walk him on his harness at service stations just in case) until we stopped in a motel overnight then he had a mad half hour running round the hotel room before settling down.