I contacted a dealership, they came up with the rim and lug nuts, mounted X-Ice, installed and checked fit, pretty easy. Just let them do their job.
Bumping this old thread as I'm looking for a set of OE lug nuts for my 16" alloys. I currently have the lugs from the steelies on there but prefer something a little dressier. Did we ever get the correct part number and thread size for alloys? Checking online, it looks like this may be the P/N: 90942-01033 201 FWIW, I'm looking for the exact nuts for the exact wheels shown here:
I got a set of gorilla nuts on Amazon a while back, to replace locking nuts. I can look it up tomorrow. They're identical except for lightly embossed number on one face.
A screen grab of my order history on amazon: Google the part number. On Amazon you can enter a vehicle in your "garage", set it as current, and it will verify if it's compatible with that vehicle. I think also, it'll be the "alloy wheel" version here: Wheels for 2015 Toyota Prius C | McGeorge Toyota Parts Genuine Toyota, and about the same price. Part number: 90942-01033 (Just searching for part number very effective, lots of genuine Toyota and aftermarket results.)
…and here's what I bought (also Gorilla) for my steelies (…shod with snows) (…package contains 20 acorn nuts plus key-lock-socket)