The Prius is 68.7 inches wide. The '78 Courier long bed is 63 inches wide, narrow enough, and low enough to hide in the Prius' aero shadow, but the bed is 59.5 inches inside, Just wide enough for a queen mattress.
When the bed is fully opened, I would erect half hoops over the whole thing, and cover it Conestoga style. Any other trailer I can find that will fit the air shadow has wheels and bearings rated for 45 to 55 mph. I built a camp trailer once on a utility trailer, drove it up to 80 mph, and blew out a wheel bearing. The courier is built for the highway.
