I know this won't work with other passengers, but we use one of these in the backseat. We have 3 dogs, 2 basset hounds and a Boxer. Any equipment/gear that can't fit on the floor boards goes either in the back of the truck or in the trailer. Our dogs are basically our kids (we don't have human children) so no way would they go anywhere but in the cab, or stay at home with a sitter. They don't live outside at home, so why would we expect them to do it when travelling?
https://www.amazon.com/TaoTronics-Waterproof-Scratch-proof-Stain-proof-approximately/dp/B019OFKASI/ref=zg_bs_3024184011_4?_encoding=UTF8&psc=1&refRID=WMTG1P969FZ1EQT1QZ8H