You can also get a refurbished utility meter from hialeahmeter.com and put it in line with the outlet (as long as nothing else plugs in). A meter and base will cost you around $50.
Yeah, I was going to second the refurbished meter thing. It is vast overkill (probably rated for 200 amps) but the refurbished meters are stupid cheap because all the utilities have swapped them with smart meters so there is virtually an unlimited supply. My solar guy said they were like $10 (they state energy credit required one on my solar). Meter bases are also stupid cheap since they are produced in insane volume. Naturally it handles 240v and is "revenue grade" (i.e. accurate).
But with that being said, something with just a CT over the wire and a monitor display would work fine as well (just probably not UL rated).
On the Kill-A-Watt - my first thought was that I would be highly suspect with running that much continuous current through it, however, it probably just has a CT in there anyway and so it is not hard to have a piece of metal of sufficient ampacity to pass the power through from the plug to the receptacle. Regardless, I probably still would not trust it (and as others have pointed out, it won't work on 240v).