Pretty much just a loop with a diameter around 4km. Was made possible by the recent update to the mesh modeler, as it allowed me to make the loop entirely super boost so you actually have the speed necessary (might not have needed that if the car wasn't limited to 1000kph).
Also thanks eyebo for calculating the shadows on this thing because my computer is too potato for the task.
Also the very top of the loop is the hardest part because the car may lose contact with the loop periodically. It isn't a very smooth loop.