Source a 16" x 16" sheet of MDF and suitable spacer material.
Secure the MDF on the baseboard with the spacer material to lift it up over the edge and the corners aligned at the rear edge of where you can cut.
Load a suitable endmill and jog it over to be centered on the left edge of the MDF and centered front–back over the edge of the front plate.
Create a pocket toolpath which will cut a slot from there to the opposite (right) edge a bit deeper than the edge of the endplate is higher than the baseboard surface
Remove the spacer material, flip the MDF over, and secure it in place.