I made one of the jigs you are talking about some time ago. You just need a straight grove to hold your shafting.
I made mine by cutting some pine with my table saw angled at 45 degrees. Did 2 them mounted them onto another piece of wood to form a V shaped grove. Got a small hand plane and started making shavings.
Start by putting your square in and take one of the corners off the full length. You just want to take the corner off. Then rotate and do the next corner.
After you take off all 4 corners, you should have an octagon shaped shaft. When you get practiced, you will have an even sided octagon.
Then take of each of these corners giving you a 16 sided shaft.
You could then usually start sanding with say 100 grit then work your way up in grits until you get where you want to go.
The idea for planning until no more wood comes off is because the plane that comes with the jigs 3Rivers sells is curved. One side of the jig is for 11/32 & the other is for 5/16.
In the 1st volume of TBB, the late Jay Massey has a good chapter on making shafting this way.