I've been thinking of doing it Erik's way for a little while but haven't got to it yet. Eventually I'll make permanent templates (in better material than MDF) that will have screw holes in them to do just that.
In the picture you showed, you're actually missing half the information. There's another clamp attached at the bottom of the neck. Those two clamps are screwed on really tight! There's nothing that's going to move them. I reposition them once or twice so I can get the whole lenght of the neck done. It goes pretty fast and the result is dead flat.
I wouldn't try that with MDF, from what I've seen, it feels like MDF has a lubricant capacity to it. It slips easy. Plus it's a bitch to work with. I love that neck blank I've been using as a straightedge I was going to use it for my next bass but now I might keep it as a fancy-pants laminated straightedge. It's 1" thick and has been jointed flat on all four sides. It's great for these applications and doesn't slip as much as MDF. At 1" thick I know the bearing is not going to miss the template and ruin my workpiece.