Hi I have been trying to learn how to add some clearance between parts that will mate.

I have an enclosure shell and a lid:

I can not figure out how to reduce the outer perimeter of the insert by about 1mm so the lid will fit comfortably into the shell. The red arrows indicate the direction I would like to shrink. 

I created a new plane, selected the edges of the shape I want to use as a mate, converted the entities, and extruded the mating shape, but I anticipate it needs some clearance where the two surfaces will contact. I have not yet saved the new entity as a separate part. I have tried making some offsets to trim a small portion, but it is not working as I hoped. 

FWIW, I just want to trim the outer perimeter, not the inner portion, of the shape.

What is the correct way to do something like this in SolidWorks?

Thank you!