The under-trailer box is a little less aesthetically challenged. It's available as an add-on; see the Spindelberger website.

FWIW, I ended up mounting my spare under the trailer (2013 model year) when I added the 15M extensions to my ASG-29. While the floor and wall material is pretty solid, it's not immune from compression damage. I ended up building a large steel doubler on the inside to spread the load of the 5 lug nuts from the spare. Even with large fender washers (before the doubler) I could see that the load from the spare would eventually cause failure in the skin. Point being, home-grown mounts for storage boxes would need to account for this and might be a bit more work than anticipated.

P3