i didn't want to use wood as it can rot from soaking up moisture, the metal platform is solid and doesn't move. you can check out some of my prius rear seat platform blog articles which might inspire you.
lol, it's really funny how people chime in for proper ventilation. Just shows how some posters don't even know how the air ducts looks like in the prius HV battery. As long as the intake vent near the door and exhaust near the 12v aux battery is ok that shouldn't be an issue.
If you don’t need the seats just pull them out. A plywood frame can be scribed out fairly simply. Lots of Prius campers have done this mod. You save a little bit on weight so that will help mileage. Added bonus. Worse case remove the seats and drive around for a week. See how you like it.