This web page describes 'cargo' versions, Rev. C, of an NHW11 03 Prius. The first was a "nap board" to replace the passenger seat. The second explains the why and how of a towing package.

Nap Board Passenger Side

On long trips, it is desirable to be able to take a 'cat nap' but the standard Prius seats do not fully recline. This is solved by taking out the passengere seat and installing a platform extending from the front floor board back to the base of the rear seat. There are 73 inches from the back of the rear seat and the front of the floor board. The rear seats remain usable and there is storage space under the platform.

Prius Towing

Transporting 4'x8' plywood on a standard Prius is possible for short distances and extremely low speeds:
This 'emergency' method is impractical and high risk. There is insufficient load security for a sudden stop or speeds above 10 mph. The risk of damaging the roof is high and it takes at least two people to properly load and secure the load.

Instead, a Coastal Electronics receiver hitch was attached and a folding, 4' x 8' trailer from Harbor Freight assembled. Speed limited to 55 mph and load limited to 1450 lbs., this is not a practical solution for interstate transportation. However, it is perfectly adequate for moving stuff around in an urban environment.