We brought our cat to a holiday in the Alps last year, which was an 8 hour journey each way. Since his regular carrier (which was really designed for cats at 5kg or less) was getting rather tight for him, therefore we built a DIY cage putting together 2 of the plastic foldable crates we got from LIDL (about 50cm long), sicuring several points with large cable fasteners and lined the bottom with a bathtowel, last used by me while working out (he loves sitting on my sweaty towel
). It was significantly roomier than the carrier, and from our shopping trips we knew these crates were very sturdy and dependable even for a 7kg cat. We had his tray handy so we could pull it out quickly for use if he had an "emergency" (it happened once on the return trip...). We didn't give him anything to eat though we was afraid of him getting sick. We placed the cage on the back seat, but in a way that he could look out between the front seat and see us, and I could turn around and fuss him (the upper flap could be opened).
The journey went well enough, he was pretty calm (except for the moment he had the nature's calling) throughout, though he has grown even further since then so we will probably have to invent something else on the next occasion...