The Prius has a drivers-side "pocket" to the left of the cargo lid as you look inside the rear hatch. Remove the pocket cover, and then remove the black plastic pocket under it by unscrewing the plastic cap on the bottom.
Look to the far left, and you'll see a black plastic assembly attached to the side of the car. That is the cabin air exhaust duct. When your vehicle is moving, this actually has some suction on it due to the way the outside end of the duct engages the moving airflow. The area around this duct will have airflow, so it's a good place to mount that 100 Watt radio.
I found that by removing the "pocket", there was enough room to put an IC-7000 in that space without interfering with anything else. There are two removable rubber plugs there that lead to the outside in that space, and you can use the holes and grommets to lead the antenna wire out to the rear of the car, if you are using my idea for mounting a screwdriver antenna with a bolt-on hitch.
The cover will still work once you've removed the pocket, but it will rest about a quarter-inch lower and you might see the need to reinforce where it sits so that some heavy cargo won't break through there. Glue some rubber blocks to support it, using a hot-melt glue gun.
This work will probably compromise the sound deadening around the pocket. Apply stick-on sound-deadening material to the entire area, and to the underside of the cover.