Quill stem diameter not matching the fork...possible to get around this?
There are products that do this, but they rely on the quill fitting inside the steerer.



You can also get threadless clamps that allow you to run a fork with no stem, and effectively DIY that product. They're mostly used by recumbent bikes that have a second pivot holding the handlebars, rather than by people not using handlebars at all.



If you're lucky you might be able to find a different quill stem that will fit inside your steerer tube. But then you have an aluminium steerer being pushed on by a steel quill in a way that the designer never thought about. That's why the commercial product clamps the quill rather than using the wedge bolt it was designed for.

But since the quill won't go into the fork steerer tube, that's never going to work. Milling out the inside the of the steerer tube, even a small amount, is risky. It's one of those "probably ok, but if it's not your face hits the road" things.