I use a double-walled bait bucket to transport fish. You can get them in most outdoor stores. The perforations in the interior lining keeps water from sloshing around too much. I sit it on my passenger floor board, or on a back floor board, then move the seat either forward or backward to wedge the bucket in place.
150g long S Amer cichlid; 90g C Amer cichlid; 90g Malawi cichlid; 40g Tanganyika cichlid; 29g "Shellie" cichlid; 29g Pea Puffer. 6,500g koi/native catfish pond; several smaller ponds