We managed to snag a top carrier just before we left using Facebook Marketplace for $40, which was an amazing deal and so worth it. We'd nearly decided not to use one because they were so expensive, but I don't know what we would have done without it.
One problem we discovered with our design was, while it was technically possible to remove the entire bed platform without dismantling its construction, the head of our bed rested on top of the spare tire compartment. We knew that if we made the spare tire extremely inaccessible, we would for sure end up with a flat--so we took the tire out and put it in the top carrier. (Murphey's Law; we never got a flat!) Also in the top carrier was our 5 gallons of spare gas, first-aid kit, tool kit, extra bedding, and an emergency bucket toilet (that Andrew only used once, and it was pretty much for fun).