Hi John,
I am going through exactly the same problem. None of the major towbar manufacturers do a bar for the NX. Lexus Sheffield have quoted me £1031 for a detachable supply and fit but with 7 pin electrics, obviously 13pin will be slightly more. They will supply Parts Only for £690 and for me to find some body to fit it but I understand the labour is quite high as they have to dismantle a lot of the car rear. Did find a company on the net in Shropshire, Telford Towbars who were listing the NX supply and fit for around £750 but have been too busy to take it any further. Holpe this helps.
Regards Clive