This was quite awhile ago. I have had my 10th gen for several years but I did end up using a dremel tool. Used a cutting wheel to make a nice rectangular cut out on both sides and then used a sanding drum to lightly round out the corners to make it look cleaner.

 

Thanks for the response. Mine was short work with the metal blade on a multi-tool. I shaved the collar smooth, all the way across, to fit a too-thick one-piece key head. I had actually bought the flip key you used on ebay, and then exchanged it, before cutting, for the key I have now, having realized it wouldn't fit.

Now I wish I'd kept the flip key, but for anyone reading, you can make either one-piece key fit this way. I would prefer the flip key because it actually requires less cutting, with a notch in the middle sufficing, leaving some of the collar there to do its job of reducing torque on the key shaft.