I've got an apparently discontinued Aluminess box on the back of my SMB. It is a top loader where the top comes completely off, and there is an "open" jerry can holder. I upgraded from my Honda 1000 generator to the 2000, but it won't fit in the box. I usually carry wood, a mat, and some odds and ends in the box. I like being able to put things out of "sight" as much as possible.
Aluminess carries two, one is the "Galley" box ($495) which just holds the generator, or the Deluxe box ($675) which can hold the generator, and with the shelf (at an extra cost) you can fit in some additional stuff.
We just took a trip down to June Lake, and I strapped the generator to the top of my current box, with a lock as extra "security" when we were away from the van. It didn't seem to have any issues with moving around, and only a minor inconvenience in getting to things in the box itself. My only question is getting some kind of cover for it when I'm out in the winter, or rainy weather. I came across this cover that looks promising http://www.ebay.com/itm/HONDA-GENERA...item25675c597d The top flap folds down so I can get the straps through and secure the generator to the top of the box, and it would be somewhat protected from the weather, but it does nothing for security past my current thick cable lock.
In looking at the Deluxe box, it seems as though it cuts out half of the view of the passenger side back window. The Galley box is about the same height as my old box, but it seems as though it would be limited to the generator only.
Any personal experiences would be appreciated.