So, it seems that Thule UK has Niro specific mounting kits on their website: Kit 4076.
It appears they recommend rapid system 753 for square bars, which is what I have. System 753 appears available from US-based retailers.
I contacted Thule USA about Niro specific kits but haven't heard back. If they don't respond soon, I'm tempted to order the mounting kit from a UK based Thule distributor.