In case you're wondering what a spool clamp is... Take a look at this picture of Bill Loveless's spool clamps from Jake Jacobson's wonderful book: Hearts & Hands. A spool clamp is basically a couple of wooden spools on either end of a carriage bolt.
Here's a shot of where I left things last night. I'd pretty much chosen to go with my hand-rail spools.
After a while I managed to get a crude circular spool shape. At this point I pushed the jig so that the spool was touching the blade and then slowly turned the carriage bolt by twisting the near end of the bolt (furthest from the blade).