Creating A Sketch Inside A Part?

Hi everyone,

I'm working on a project that requires me to create a feature inside a shelled out extrusion, and I thought that I could use section view to get a slice of the extrusion and create my sketch with a plane, but I need the sketch to adhere to the sides of the shelled extrusion, which is impossible since a sketch isn't actually made on the extrusion, but the plane.  Is there anyway I can make a sketch on a section view surface inside that shelled extrusion?

Thanks. 

SolidworksParts And Features