not sure on the numbers, just throwing a random ass idea out - but we went on a road trip last year and rented a hardshell rooftop cargo unit (ie Thule). You could prob grab a few of those for a few grand, install should be pretty easy, they’re pretty durable / low maintenance. i’d honestly rent one again. we dont need one frequently enough to justify buying one so the price was still decent enough considering how much more comfortable we were in the vehicle with extra space.