Here are some pictures of the jig. It has 3 pieces. Two brass bushings and an aluminum sleeve with a cutout. The brass sleeves serve double duty. After you've glued up your cork rings you can clamp the rough insert between the brass bushings on a piece of 1/4" allthread. Then just turn down the insert until it's flush with the brass.

Then you slide the insert into the aluminum sleeve and clamp it in place with the brass bushings. The compression of the insert lengthwise causes the cork to expand slightly into the aluminum and it stays in place pretty well. Then you just sand the cork down until it's flush with the aluminum cut out. If you like, you could make a sanding block with a slight radius cut into it so you can make the rounded style mortise (like a thumbnail bit would make) instead of just the flat one.