The board won't slide if you'll use small nails before clamping - two on 3rd fret and two on 12 (or any other). After clamping you can easily put them away. Sometimes it's not possible - for example if you use big plate to clamp the board. In this case u can use toothpicks insead so the won't jut out from the fret slot.
So, i slot, taper, clamp and then making radius.