7. Recut the gap side so the gap is a little less high than the exposed height of the blade runner.
8. Use a hole saw with the ROH radius to cut out that circle, using a drill press with a vice to hold the block in place. The cut hole will be your sanding drum. It will fit the hole somewhat loosely, to make our tool.
8.5 Recut the other sides so the tool isn't too large.
9. On the side opposite that gap, near the center length use a self tapping screw to create a thread (or: can a tap and die kit cut wood?) deep enough to go just past the circle. Replace the screw with a bolt with the same thread. Use it to press the sanding drum against the side of the hole next to the gap, locking the sandpaper in place while you use the tool.
10. Cut sandpaper, loosen the bolt, and insert sandpaper. It should be long enough to extend most of the way to either side of the bolt. If it doesn't extend past the hole, the sanding drum won't center accurately on the hole. Tighten the bolt.
11. Cut tape that is a bit wider than the gap is high, to a length longer than the tool is long. I use Scotch tape on my Pro-filer. Pull tape taut, insert into the gap, touching the sanding drum so it aligns parallel, and pulled against the side of the gap to make it stick. Stick the extra tape to the ends and sides of the tool. Repeat on the other side of the gap. Briefly insert the blade and push against the tape hard on each side, to make it stick better. Add more layers until the blade slides through snugly.
12. Re-insert sanding drum and sandpaper, tighten the bolt, and sharpen a cheap discarded blade, like on the old skates that ice rinks throw away. The inside edge will be longer than the outside edge, or vice versa, because the gap probably isn't exactly centered on the hole.
13. So: Discard the tape and retape, using more layers to one side, iterating step 10 & 11 until it is properly centered.
Thoughts, ideas for improvement?