Do you have a long ruler, if so, have someone else hold it down on the line you drew out, so it will guide your knife.
However, another option for you, is when you draw out your lines for the removal of the balsa / foam strip that is 1" wide (on all three surfaces), just go down the middle with the knife. Then you can finish it off with the band saw and using the shuck (meaning, the removal of the foam/balsa at the border of the foam and LE with the TE (1" wide area). The advantage of the band saw is that it will guarantee that your cut will be square. I would recommend you use a band saw. I do not know how good you are with your hands, but even I tried to split down the middle with a knife, and I cam very close (on the opposite side) to violate the into the line of the hinge gap cut.