Stephen Shepherd
Make a board with a rebate/rabbit the thickness of your drawer sides, but with a bit of a taper. Then turn it upside down and hold it fast to your bench with a holdfast.

Simply jamb your pieces under the rebate and against a dog in one of your existing holes. The only time you need to move the board is for different widths of drawer sides, so the fence of the plow plane is over the edge of the bench.

Stephen