I bought an ammo box and mounted it to the left side and then painted to match. I attached it with an "L" bracket that I rebent to get the correct angle. I didn't want to mount a tool box directly behind the seat as this would block the view when connecting implements to the hitch and the SMV.
BTW the new Branson/Montana 3510 models (non-hydro) I saw at the Sunbelt Ag Expo last week come standard with a "toolbox" just forward of the left step. The size appears to be able to hold a checkbook and a pencil and not much more.