NC419N, with one reply so far, looks like you're going to have to be the pioneer on this once.

When you get it figured out, let the rest of us know. I've noodled this for awhile and places to mount a toolbox are hard to come up with on the NX cabbed models. For a very small box (for hitch and link pins and a drawbar pin) the backing plate for the rear windshield wiper reservoir would work. For anything heavier, you'd have to reinforce it with gussets. But before you commit to that, take a look at EricTheOracle's nifty chainsaw carrier that he designed and mounted to that backing plate on his NX6010HST Cab. You may prefer to use the location for that rather than a toolbox. For small stuff like pins, even wrenches and other small tools, a 15" soft-sided canvas tool bag from HF or similar fits nicely behind the factory seat. I have two, one sits on top of the other, and they hold a lot of stuff. HF sells two types. The one with the soft bottom fits best for this.
15" Canvas Tool Bag