Bought a similar product just not rotopax not mounted yet have carried on the floor passenger side and bungeed to rack. I think it would be cool to make a sliding storage drawer under drivers seat to store stuff like a rotopax there plus other Keerap. In that way you could also lock it.
I have the 2.5 gal. rotopax set up that is laying down on a platform made of plywood that I have attached to the rear rack . You can get just about any type of mount you can think of. Pricey but a very good quality product.