Make the FILL a seperate object.

Hi,

I'm new to Illustator and wondered if there was an easier way to do this.

After making a shape and applying a fill, is there a way to make just the fill an object in it's own right.

The image below gives an example of what I'm trying to acheive.

I start off with the box on the left and apply the red fill,

I want to end up with 2 objects, as seen at the right of the image, the box outline, and the red fill as 2 seperate entities. The dimensions are critical and the red box, must always fit exactly inside the black box.

At the moment, I'm making two copies of the original box, then removing the fill on one, to give me the black outline, then on the other removing the stroke and then reducing the size of the red box down, so it fits in the black box again.

Time comsuming, and I need to do this for several hundred shapes of all different sizes and stroke widths etc, so it's going to take a very long time.

Is there a way just to select the fill on it's own and make it into a seperate object.

I'm very new, so just worried I've missed something obvious.

Thank you for any help