I use the x-acto blades which look like a "hoop". Can't remember if they are called a "gouge" or "Router", or something else!
They come in two sizes, one about 1/4" od, and the other about 1/2" od. Both sharp on one side. Even available in rectangular and triangular shapes.
Finish up the inside with a sanding block made from a dowel or piece of LE stock like from an old Ringmaster (my favorite!).