Joe; you could also enlarge the upper cutout portion (maybe up to an 1" bigger) and glue in 1 1/4' to 1 1/2' filler pieces, then re-rout the hole accurately. Leave the lower shoulder portion alone.
Rather than make the two side pieces exactly the right length, just make all the pieces maybe 4" -6" too long and just run them consecutively around the perimeter with only one end butted up squarely to the previous one...sort of pinwheel style. (Sorry if that doesn't make sense, Tom (DRT) has an illustration he posts occasionally. Stick does as well.)