I was wanting to do this as well, I want it to be transverse when in use, but I want to be able to remove it to allow the aux motor to be fitted if required. When the box is full of gear it will be heavy though. U bolts fitted to the deck, then ratchet straps to secure, seem the best option. In my last craft, I had my aux strapped this way, and it never budged. The rathchet straps need attention due to corrosion though.
I was hoping the box could be used as a seat as well along these lines
Fixing the straps to the box is the next problem. Being that far aft, bouncing about is not such an issue.