Cargo Box/Tray

Hello everyone,
Looking to take a road trip soon with 3 people and we’ve decided to take the Prius. We’re heading down to Florida soon to do disaster relief so we definitely won’t be packing light (we’ll need to bring bedding and enough clothes to last for 2 weeks of yucky work, plus a large backpack-sized camera bag). Anybody made a trip like this in the Prius? Looking for recommendations on a roof top or hitch cargo box/rack. Probably something that will need to be waterproof. I’d want to be able to see out the rear the whole time since visibility already isn’t exceptional. Hoping for a solution for a couple hundred or so (I see a lot of stuff in the $500 range and I’d like to stay away from that if possible. Also my hitch has rusted out as well so I’d need a new one if it’s something hitch-mounted, any recommends for a good hitch? What does mpg look like with a roof rack box vs something hanging off the back of the car? Thanks in advance!