Going through a 2 by 6 with a knife is gonna get you a lot of frustrating exercise. If you have a drill, power that is, you can mark out the edge of the hold with tape or a string with a nail at each end that's the radius of your hole.
then drill 1/4 inch holes around the edge just inside the mark. Now you should be able to break out the remaining wood with any size gouge you've got on hand. Not a nice clean result but it'll work. tom
