Well I look at this as static, as there is no repeat applied to insure a fluid nature. As you stated the l/r are fixed and the center is one image.

I suggest that you combine the 3 elements into one and apply via the css as described changing to leftBoxHeader & right. then apply a border with padding to get the outline spaced as you want. This will be square corners for now,

You can download the NiftyZen template in the downloads section to see the code for the rounded corners that you would have to re-apply for the sidebox borders. Or Google for the term nifty corners for the raw code.

Or you can retain the current sidebox and just apply a single image to the center area. Your choice