time to go home, but here's a few

8/there may be a chance that the round file clamped w/ 2 separated one inch spring clamps w/ the jaws overhanging (by say 1/2in) would slide back and forth w/ the jaws as the guides - real simple

9/ same thing w/ the sandpaper wrapped dowel with the benefit that the clamps also clamp the sandpaper hehehe

there's a bunch more but not as great and time to go

-mick