There is a bolt-on hitch kit available for the Prius that you can bolt onto the bottom of the body. I think there are some tie-downs that you remove to put this thing on. Then you can use a normal hitch mount.

Note this does not mean you can now pull a 28' camping trailer (or any trailer, really) with the Prius. It's made specifically for a hitch rack.

It'll also cut your mileage by 20% or so.