I'm just going off the reports that NK has 5-10 rudementary nuclear weapons. Given that, the tracking of uranium doesn't matter much. If NK already has a weapon, they have already lept over the first big hurdle. Now, their major problem would be getting it the the US. NK clearly doesn't have a bomber or missile that would reach the US, so they would have to find some way to get it here invisibly. It's been a while since I did HazMat international shipping, but the main defenses are at areas of entry. Containers generally aren't inspected on ship, but after being unloaded.
You're right that a 53' container from Pyongyang is going to raise some eyebrows, so the smart play would be to get point of origin somewhere along the line. I am sure that for the right price some East African country would be happy to oblige. NK could also probably just forge BoL's from Angola or Mozambique. While I am sure that we eyeball any ship leaving NK and probably inspect anything that arrives from NK onboard, I'd be highly surprised if we keep eyeballs on them the entire way here.
I don't see much, if anything that would keep a container ship from grounding itself on the south shore of Manhattan, say near the seaport. It'd obviously be a one-way mission and the cost of the ship is a fairly large entry barrier, but a scenario such as the one I described seems to be the avenue with the greatest chance of success if NK really wanted to put a nuke on US soil. Hell, a luxury yacht would be big enough to hold bombs such as the ones we dropped on Japan. It wouldn't even need to be a full-sized container ship...though a trans-pacific crossing is much higher risk.