I've built a few quill stems. After cutting the angle of the quill I'd braze a short length of tubing inside and cut it again, so the wedge had a good surface to push against. I don't recall the details, I guess we were using 7/8" 16 ga. seamless for the quill and 3/4" for the insert. Then a 1" collar at the top to give lots of area for attaching the extension, which was probably an offcut of 531 seat tube notched and squashed to fit the quill, and similarly at the clamp end. These were not lightweight but they were strong, they were for specific use cases like my girlfriend who needed a very short stem, my experiments with funny handlebars (a few tandem stoker stems too, but they weren't quills obviously).