I know this would require some investment, but have you considered installing a trailer hitch? It would open up a number of possibilities for storage solutions, and would also offer a recovery point if you are tackling tougher terrain that you might get stuck on. Even then I might still opt to store a jerry can on the roof to avoid issues if you were rear ended, but you could move some of your other boxes to a hitch basket like this one:

https://www.amazon.com/Hitch-Mount-Cargo-Carrier-Basket/dp/B07TT9TRXR/